Add build variables required to roll ANGLE (#565)

This lands buildroot changes required to roll ANGLE to tip-of-tree;
specifically, to commit 866210add2f6a2f891306c1d3d802a558462c07b.

List of relevant ANGLE commits and the associated change to our
buildroot:
* 45344f05555eb7fc5a907d45ebc7a7d7e05856f4: dependency added on
  `is_chromecast`.
* 08805e351f3cbfacb515557dc7f7f3d9d57e93bd: dependency added on
  `//build/win:runtime_libs`.
* b62f8dd1164a7486646ad9281ffd6f02aa926e1c: dependency added on
  `is_chromeos_lacros`.
* 81f03cc092f9122bfa574241c28ea7d8c1ff1617: dependencies added on
  `angle_spirv_headers_dir` and `angle_spirv_tools_dir`.
* 96aa681ce39d288a58692debacb6dc2107971c07: dependency added on
  `ozone_platform_headless`.
* 48da1c3594095fe363bd18e58982d9856f6654fa: dependency added on
  `build_overrides/swiftshader.gni`.
* fa2e37497495b1215062603e8d214c2f0ba79451: dependency added on
  `ozone_platform_x11`.
* 89e3de9720ced82ad9ee7fbaf11a6a1ca2aa2349: dependency added on
  `using_sanitizer`
* 2be4f04a47c8b30e5a4cfaa5f4ca5acff68b717c: dependency added on
  `ozone_platform_wayland`.

Issue: flutter/flutter#102117

Author: cbracken

build/config/BUILDCONFIG.gn

@@ -286,6 +286,13 @@ is_apple = is_ios || is_mac
 # Needed for some third_party build files from Chromium.
 is_nacl = false
 
+# Needed for //third_party/angle.
+is_chromecast = false
+is_chromeos_lacros = false
+ozone_platform_headless = false
+ozone_platform_x11 = false
+ozone_platform_wayland = false
+
 default_library_type = "static_library"
 
 # =============================================================================
@@ -294,7 +301,8 @@ default_library_type = "static_library"
 
 # These Sanitizers all imply using the Clang compiler. On Windows they either
 # don't work or work differently.
-if (!is_clang && (is_asan || is_lsan || is_tsan || is_msan)) {
+using_sanitizer = !is_win && (is_asan || is_lsan || is_tsan || is_msan)
+if (!is_clang && using_sanitizer) {
   is_clang = true
 }
 

build/config/compiler/BUILD.gn

@@ -47,10 +47,6 @@ config("default_include_dirs") {
   ]
 }
 
-if (!is_win) {
-  using_sanitizer = is_asan || is_lsan || is_tsan || is_msan
-}
-
 # compiler ---------------------------------------------------------------------
 #
 # Base compiler configuration.

build/win/BUILD.gn

@@ -0,0 +1,12 @@
+# Copyright 2019 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Windows DLLs on which Flutter has a runtime dependency that should be shipped
+# with our distributable binaries.
+#
+# This target is for compatibility with the Chromium buildroot. In particular,
+# //third_party/angle depends on it.
+group("runtime_libs") {
+  data = []
+}

build_overrides/angle.gni

@@ -7,18 +7,20 @@ import("//build/toolchain/toolchain.gni")
 
 # The ANGLE build requires this file to point to the location of third-party
 # dependencies.
-
 angle_root = "//third_party/angle"
 
 # Flutter's buildroot looks enough like Chromium to satisfy Angle, and enough
 # to cause GN variable collisions if we don't set this.
 angle_has_build = true
 
+# Overrides for ANGLE's dependencies.
 angle_googletest_dir = "//third_party/googletest/src"
-angle_libpng_dir = "//third_party/libpng"
 # Note: This path doesn't actually exist; see
 # //build/secondary/third_party/jsoncpp/BUILD.gn
 angle_jsoncpp_dir = "//third_party/jsoncpp"
+angle_libpng_dir = "//third_party/libpng"
+angle_spirv_headers_dir = "//third_party/spirv_headers"
+angle_spirv_tools_dir = "//third_party/spirv_tools_flutter"
 
 # This is a general Chromium flag, but in the Flutter build only ANGLE needs it
 # so it is defined here.

build_overrides/spirv_tools.gni

@@ -0,0 +1,10 @@
+# Copyright 2019 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# We are building inside Flutter.
+spirv_tools_standalone = false
+
+# Paths to SPIRV-Tools dependencies in Flutter.
+spirv_tools_googletest_dir = "//third_party/googletest"
+spirv_tools_spirv_headers_dir = "//third_party/spirv_headers"

build_overrides/swiftshader.gni

@@ -0,0 +1,9 @@
+# Copyright 2019 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# We are building SwiftShader in Flutter.
+swiftshader_standalone = false
+
+# Path to SwiftShader.
+swiftshader_dir = "//third_party/swiftshader"