Android: make the top level Android makefile smarter.

Instead of requiring an environment variable to be set, which has
resulted in issues with things not being rebuilt correctly when the
variable is forgotten, check to see if the gyp-generated Android
makefile has been created, and if so assume we are doing a WebView
build. This file only exists if gyp has been run with
GYP_GENERATORS=android which only happens in the WebView tree.

Also, rename CHROMIUM_WEBVIEW_DIR to CHROMIUM_DIR as the directory
itself is not webview-specific; it's just the Chromium tree root.

BUG=

Review URL: https://codereview.chromium.org/11269007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@164291 0039d316-1c4b-4281-b951-d872f2087c98

Android.mk

@@ -8,8 +8,11 @@
 # which prevents the Android build system from mistakenly loading any other
 # Android.mk that may exist in the Chromium tree.
 
-ifdef CHROME_ANDROID_BUILD_WEBVIEW
-CHROMIUM_WEBVIEW_DIR := $(call my-dir)
-include $(CHROMIUM_WEBVIEW_DIR)/GypAndroid.mk
-include $(CHROMIUM_WEBVIEW_DIR)/android_webview/Android.mk
+CHROMIUM_DIR := $(call my-dir)
+
+# Assume that if the gyp autogenerated makefile exists, we are doing the
+# WebView build using the Android build system.
+ifneq (,$(wildcard $(CHROMIUM_DIR)/GypAndroid.mk))
+include $(CHROMIUM_DIR)/GypAndroid.mk
+include $(CHROMIUM_DIR)/android_webview/Android.mk
 endif