Share precompiled header setup for blink between Windows and 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 ca5d37ceb669962f2b942391e9c90c4b2f0c26aa..8e148f2381a6d42e589067675d695d291926fef0 100644
--- a/third_party/WebKit/Source/BUILD.gn
+++ b/third_party/WebKit/Source/BUILD.gn
@@ -69,6 +69,8 @@ config("blink_pch") {
 
       # Force include the header.
       cflags = [ "/FI$precompiled_header" ]
+    } else if (is_mac) {
+      precompiled_source = "//third_party/WebKit/Source/build/mac/Prefix.h"
     }
   }
 }
@@ -163,11 +165,3 @@ 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"
-  }
-}