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

mojo/public/cpp/bindings/tests/string_unittest.cc

 // 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.
 
 #include <sstream>
 
+#include "gtest/gtest.h"
 #include "mojo/public/cpp/bindings/string.h"
-#include "testing/gtest/include/gtest/gtest.h"
 
 namespace mojo {
 namespace test {
 
 TEST(StringTest, DefaultIsNull) {
   String s;
   EXPECT_TRUE(s.is_null());
 }
 
 TEST(StringTest, ConstructedWithNull) {
@@ skipping 103 matching lines @@
   String s("abc");
   String null;
 
   std::ostringstream so;
   so << "s=" << s << ", null=" << null;
   EXPECT_EQ("s=abc, null=", so.str());
 }
 
 }  // namespace test
 }  // namespace mojo