Add precompiled headers to GN build for large targets.

ppapi/cpp/BUILD.gn

 # Copyright 2015 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("//ppapi/ppapi_sources.gni")
 
 # Link to this target to get the PPAPI C++ wrapper objects and plugin startup
 # code. See also "objects" below.
 source_set("cpp") {
   sources = [
     "module_embedder.h",
     "ppp_entrypoints.cc",
   ]
 
   deps = [
     ":objects",
     "//ppapi/c",
   ]
 }
 
 # Link to this target to get only the PPAPI C++ wrapper objects but not the
 # plugin startup code. Some plugins need special startup code that they supply
 # themselves.
 source_set("objects") {
   sources = rebase_path(ppapi_sources.cpp_source_files, ".", "..")
+  configs += [ "//build/config:precompiled_headers" ]
 }