[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..33410f246d689c65be47d7b4084f0e97ca9def8c 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 +86,11 @@ config("non_test_config") {
     cflags += [ "-Wglobal-constructors" ]
   }
 }
+
+if (is_mac) {
+  # This sets up precompiled headers for Mac.
+  config("mac_precompiled_headers") {
+    precompiled_header = rebase_path("build/mac/Prefix.h", root_build_dir)
+    precompiled_source = "//third_party/WebKit/Source/build/mac/Prefix.h"
+  }
+}