mojo/public: Include gtest.h as "gtest/gtest.h", instead of via "absolute" path.

mojo/public/cpp/utility/tests/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("../../../mojo_sdk.gni")
 
 mojo_sdk_source_set("tests") {
   testonly = true
 
   sources = [
     "run_loop_unittest.cc",
   ]
 
-  deps = [
-    "//testing/gtest",
-  ]
-
   mojo_sdk_deps = [
+    "mojo/public:gtest",
     "mojo/public/cpp/system",
     "mojo/public/cpp/test_support:test_utils",
     "mojo/public/cpp/utility",
   ]
 }