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 7766c1891d2896f42e875717d45a7b438e6642b5..99ab7d820151c17dbdd511ccea6c890e8b0aba41 100644
--- a/third_party/WebKit/Source/core/BUILD.gn
+++ b/third_party/WebKit/Source/core/BUILD.gn
@@ -48,6 +48,41 @@ config("core_include_dirs") {
   }
 }
 
+import("//build/config/pch.gni")
+
+config("blink_core_pch") {
+  if (enable_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" ]
+
 source_set("generated") {
   deps = [
     ":core_generated",
@@ -201,8 +236,9 @@ component("core") {
 
 source_set("testing") {
   configs += [
-    "//third_party/WebKit/Source:inside_blink",
+    ":blink_core_pch",
     "//third_party/WebKit/Source:config",
+    "//third_party/WebKit/Source:inside_blink",
   ]
 
   deps = [
@@ -1315,6 +1351,7 @@ source_set("unit_tests") {
   ]
 
   configs += [
+    ":blink_core_pch",
     "//third_party/WebKit/Source:config",
     "//third_party/WebKit/Source:inside_blink",
   ]