Adding target to build SwiftShader

Added the swiftshader target to BUILD.gn's deps in order to enable building SwiftShader using 'ninja -C out/Default swiftshader' or similar command. The 'swiftshader' target already existed in third_party/swiftshader/BUILD.gn, but was unaccessible due to not being referenced in any other BUILD.gn files. Note that SwiftShader does not automatically build with Chromium.

BUG=630728

Review-Url: https://codereview.chromium.org/2330173002
Cr-Commit-Position: refs/heads/master@{#418934}

BUILD.gn

@@ -500,6 +500,10 @@ group("both_gn_and_gyp") {
     ]
   }
 
+  if (is_linux || is_win) {
+    deps += [ "//third_party/swiftshader" ]
+  }
+
   # TODO(GYP): Figure out which of these should (and can) build
   # for chromeos/ios.
   if (!is_chromeos && !is_ios) {