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

mojo/public/cpp/system/tests/buffer_unittest.cc

 // Copyright 2016 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.
 
 // This file tests the C++ wrappers in mojo/public/cpp/system/buffer.h.
 
 #include "mojo/public/cpp/system/buffer.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "gtest/gtest.h"
 
 namespace mojo {
 namespace {
 
 TEST(BufferTest, BasicSharedBuffer) {
   // Cursory compilation test of |MakeScopedHandle()| with shared buffer
   // handles.
   EXPECT_FALSE(MakeScopedHandle(SharedBufferHandle()).is_valid());
 
   ScopedSharedBufferHandle shared_buffer;
@@ skipping 11 matching lines @@
   shared_buffer.reset();
 
   // Can still write to it even after the handle is closed.
   *static_cast<char*>(pointer) = 'y';
 
   EXPECT_EQ(MOJO_RESULT_OK, UnmapBuffer(pointer));
 }
 
 }  // namespace
 }  // namespace mojo