[GN]: Enable PCH for WebKit on Mac.

third_party/WebKit/Source/BUILD.gn

Index: third_party/WebKit/Source/BUILD.gn
diff --git a/third_party/WebKit/Source/BUILD.gn b/third_party/WebKit/Source/BUILD.gn
index 370cde3c5f91a8ffad945a4454506c6e8973ac73..232d569490ab3dbc8e140fbde6714b8850fd72af 100644
--- a/third_party/WebKit/Source/BUILD.gn
+++ b/third_party/WebKit/Source/BUILD.gn
@@ -27,7 +27,10 @@ config("inside_blink") {
 # config -----------------------------------------------------------------------
 
 config("config") {
-  include_dirs = [ ".", ".." ]
+  include_dirs = [
+    ".",
+    "..",
+  ]
 
   cflags = []
   defines = []
@@ -83,3 +86,10 @@ config("non_test_config") {
     cflags += [ "-Wglobal-constructors" ]
   }
 }
+
+# Some Mac-specific parts of WebKit won't compile without having this prefix
+# header injected.
+config("mac_prefix_header") {

[brettw, 2015/10/12 16:47:27]

It seems like this really turns on precompiled headers rather than just
inserting a prefix. It seems like the naming and the comment are misleading
here.

Also, if this is mac-only, can we wrap this definition in an is_mac condition?

[Bons, 2015/10/12 17:11:16]

Done.

+  precompiled_header = rebase_path("build/mac/Prefix.h", root_build_dir)
+  precompiled_source = "//third_party/WebKit/Source/build/mac/Prefix.h"
+}