Share precompiled header setup for blink between Windows and Mac.

third_party/WebKit/Source/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("config.gni")
 if (is_clang) {
   import("//build/config/clang/clang.gni")
 }
 
 visibility = [ "//third_party/WebKit/*" ]
@@ skipping 51 matching lines @@
       precompiled_header = rebase_path("build/win/Precompile.h", root_build_dir)
 
       # This is a file that GN will compile with the above header. It will be
       # implicitly added to the sources (potentially multiple times, with one
       # variant for each language used in the target).
       precompiled_source =
           "//third_party/WebKit/Source/build/win/Precompile.cpp"
 
       # Force include the header.
       cflags = [ "/FI$precompiled_header" ]
+    } else if (is_mac) {
+      precompiled_header = rebase_path("build/mac/Prefix.h", root_build_dir)

[Nico, 2017/03/03 16:04:35]

I don't think you need the rebase_path call here. Maybe try removing it and see
if that hurts. (I'm aware you're just moving this around)

(`gn help precompiled_header` is pretty confusing though, I read through gn's
code to understand how this is used)

[Nico, 2017/03/03 19:28:38]

Looking at this more, maybe precompiled_header isn't used at all with gcc-style
pchs -- does removing this line completely make any difference?

[Nico, 2017/03/03 20:30:31]

I think you can in fact remove this line.
https://codereview.chromium.org/2734523002/
https://codereview.chromium.org/2731753003/

+      precompiled_source = "//third_party/WebKit/Source/build/mac/Prefix.h"
     }
   }
 }
 
 # config -----------------------------------------------------------------------
 
 config("config") {
   include_dirs = [
     ".",
     "..",
@@ skipping 74 matching lines @@
 
 # The follow configs apply to all targets except for unit tests, which rely on
 # static initializers.
 config("non_test_config") {
   cflags = []
 
   if (is_clang) {
     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"
-  }
-}