[GN]: Enable PCH for WebKit on Mac.

third_party/WebKit/Source/platform/BUILD.gn

 # 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.
 
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 import("//third_party/WebKit/Source/build/scripts/scripts.gni")
 import("//third_party/WebKit/Source/config.gni")
 import("//third_party/WebKit/Source/platform/platform.gni")
 import("//third_party/WebKit/Source/platform/platform_generated.gni")
@@ skipping 217 matching lines @@
 
       # Mac uses only ScrollAnimatorMac.
       "scroll/ScrollbarThemeNonMacCommon.cpp",
       "scroll/ScrollbarThemeNonMacCommon.h",
 
       # Uses LocaleMac instead.
       "text/LocaleICU.cpp",
       "text/LocaleICU.h",
     ]
 
-    # Some Mac-specific parts of WebKit won't compile without having this
-    # prefix header injected.
-    cflags = [
-      "-include",
-      rebase_path("../build/mac/Prefix.h", root_build_dir),
+    defines += [ "WebFontCache=ChromiumWebCoreObjCWebFontCache" ]
+    configs += [
+      ":mac_objc_renaming",
+      "//third_party/WebKit/Source:mac_precompiled_headers",
     ]
-
-    defines += [ "WebFontCache=ChromiumWebCoreObjCWebFontCache" ]
-    configs += [ ":mac_objc_renaming" ]
     libs = [
       "Accelerate.framework",
       "Carbon.framework",
       "Foundation.framework",
     ]
   } else {
     sources -= [
       "geometry/cg/FloatPointCG.cpp",
       "geometry/cg/FloatRectCG.cpp",
       "geometry/cg/FloatSizeCG.cpp",
@@ skipping 166 matching lines @@
 }
 
 if (current_cpu == "x86" || current_cpu == "x64") {
   source_set("blink_x86_sse") {
     sources = blink_platform_sse_files
     deps = [
       ":blink_common",
     ]
   }
 }