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

mojo/public/cpp/system/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 = [
     "buffer_unittest.cc",
     "data_pipe_unittest.cc",
     "handle_unittest.cc",
     "macros_unittest.cc",
     "message_pipe_unittest.cc",
     "time_unittest.cc",
     "wait_unittest.cc",
   ]
 
-  deps = [
-    "//testing/gtest",
+  mojo_sdk_deps = [
+    "mojo/public:gtest",
+    "mojo/public/cpp/system",
   ]
-
-  mojo_sdk_deps = [ "mojo/public/cpp/system" ]
 }