Share precompiled header setup for blink between Windows and Mac.

third_party/WebKit/Source/platform/BUILD.gn

Index: third_party/WebKit/Source/platform/BUILD.gn
diff --git a/third_party/WebKit/Source/platform/BUILD.gn b/third_party/WebKit/Source/platform/BUILD.gn
index a7bae8a220da1a0f47e31d384d1324fdcf61edfe..8f51e0ebc89f61e8f32a3aca18f82204525daeee 100644
--- a/third_party/WebKit/Source/platform/BUILD.gn
+++ b/third_party/WebKit/Source/platform/BUILD.gn
@@ -180,6 +180,11 @@ config("blink_platform_pch") {
 
       # Force include the header.
       cflags = [ "/FI$precompiled_header" ]
+    } else if (is_mac) {
+      precompiled_header =
+          rebase_path("mac/Precompile-platform.h", root_build_dir)
+      precompiled_source =
+          "//third_party/WebKit/Source/platform/mac/Precompile-platform.h"
     }
   }
 }
@@ -1460,7 +1465,6 @@ component("platform") {
       "text/LocaleICU.h",
     ]
 
-    configs += [ "//third_party/WebKit/Source:mac_precompiled_headers" ]
     libs = [
       "AppKit.framework",
       "Accelerate.framework",