Enable precompiled headers for Blink on Windows

third_party/WebKit/Source/core/BUILD.gn

Index: third_party/WebKit/Source/core/BUILD.gn
diff --git a/third_party/WebKit/Source/core/BUILD.gn b/third_party/WebKit/Source/core/BUILD.gn
index 3f191eb4fc5b69f40a1a53143ef5a3e4c06a262b..8eff051a5a19c9442ce166b3ffd2219f413ec968 100644
--- a/third_party/WebKit/Source/core/BUILD.gn
+++ b/third_party/WebKit/Source/core/BUILD.gn
@@ -1,6 +1,6 @@
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
+# found in the LICENSE file.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/ui.gni")
@@ -64,6 +64,41 @@ config("core_include_dirs") {
   }
 }
 
+import("//build/config/pch.gni")
+
+config("blink_core_pch") {
+  if (!disable_precompiled_headers) {
+    if (is_win) {
+      # This is a string rather than a file GN knows about. It has to match
+      # exactly what's in the /FI flag below, and what might appear in the
+      # source code in quotes for an #include directive.
+      precompiled_header = rebase_path("win/Precompile-core.h", root_build_dir)
+
+      # This is a file that GN will compile with the above header. It will be
+      # implicitly added to the sources (potentially multiple times, with one
+      # variant for each language used in the target).
+      precompiled_source =
+          "//third_party/WebKit/Source/core/win/Precompile-core.cpp"
+
+      # Force include the header.
+      cflags = [ "/FI$precompiled_header" ]
+
+      # Disable warning for "this file was empty after preprocessing". This
+      # error is generated only in C mode for ANSI compatibility. It conflicts
+      # with precompiled headers since the source file that's "compiled" for
+      # making the precompiled header is empty.
+      #
+      # This error doesn't happen every time. In VS2013, it seems if the .pch
+      # file doesn't exist, no error will be generated (probably MS tested this
+      # case but forgot the other one?). To reproduce this error, do a build,
+      # then delete the precompile.c.obj file, then build again.
+      cflags_c = [ "/wd4206" ]
+    }
+  }
+}
+
+core_config_add += [ ":blink_core_pch" ]
+
 # GYP version: WebKit/Source/core/core.gyp:webcore_generated
 source_set("generated") {
   deps = [
@@ -288,6 +323,7 @@ source_set("testing") {
   configs += [
     "//third_party/WebKit/Source:inside_blink",
     "//third_party/WebKit/Source:config",
+    ":blink_core_pch",
   ]
 
   deps = [
@@ -498,7 +534,7 @@ source_set("core_generated") {
   }
 }
 
-# core_bindings_generated ------------------------------------------------------
+# core_bindings_generated ------------------------------------------------------
 
 # GYP version: WebKit/Source/core/core_generated.gyp:core_event_interfaces
 generate_event_interfaces("core_event_interfaces") {
@@ -506,7 +542,7 @@ generate_event_interfaces("core_event_interfaces") {
   output_file = "core/EventInterfaces.in"
 }
 
-# generated_testing_idls -------------------------------------------------------
+# generated_testing_idls -------------------------------------------------------
 
 # GYP version: WebKit/Source/core/core_generated.gyp:generated_testing_idls
 group("generated_testing_idls") {
@@ -583,7 +619,7 @@ action("generated_testing_idls_internal_runtime_flags") {
   ]
 }
 
-# make_core_generated ----------------------------------------------------------
+# make_core_generated ----------------------------------------------------------
 
 # GYP version: WebKit/Source/core/core_generated.gyp:make_core_generated
 group("make_core_generated") {
@@ -770,7 +806,7 @@ process_in_files("make_core_generated_svg_element_type_helpers") {
   ]
 }
 
-# make_event_factory -----------------------------------------------------------
+# make_event_factory -----------------------------------------------------------
 
 # "EventFactory" in make_core_generated from GYP.
 process_in_files("make_core_generated_event_factory") {
@@ -787,7 +823,7 @@ process_in_files("make_core_generated_event_factory") {
   ]
 }
 
-# make_names -------------------------------------------------------------------
+# make_names -------------------------------------------------------------------
 
 # "MediaFeatureNames" in make_core_generated from GYP.
 process_in_files("make_core_generated_media_feature_names") {
@@ -863,7 +899,7 @@ make_names("make_core_generated_input_type_names") {
   ]
 }
 
-# make_qualified_names ---------------------------------------------------------
+# make_qualified_names ---------------------------------------------------------
 
 # "MathMLNames" in make_core_generated from GYP.
 make_qualified_names("make_core_generated_math_ml_names") {
@@ -904,7 +940,7 @@ make_qualified_names("make_core_generated_xml_names") {
   ]
 }
 
-# make_token_matcher -----------------------------------------------------------
+# make_token_matcher -----------------------------------------------------------
 
 # "MakeTokenMatcherForViewport" in make_core_generated from GYP.
 make_token_matcher("make_core_generated_make_token_matcher_for_viewport") {
@@ -912,7 +948,7 @@ make_token_matcher("make_core_generated_make_token_matcher_for_viewport") {
   output_file = "$blink_core_output_dir/HTMLMetaElement.cpp"
 }
 
-# One-off scripts --------------------------------------------------------------
+# One-off scripts --------------------------------------------------------------
 
 # "generatePrivateScript" in make_core_generated from GYP.
 # FIXME: The implementation of Blink-in-JS is not yet mature.