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

mojo/public/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.gni")
 
 group("public") {
   # Meta-target, don't link into production code.
   testonly = true
   deps = [
@@ skipping 38 matching lines @@
     deps += [ "interfaces/network" ]
   }
 }
 
 static_library("libmojo_sdk") {
   complete_static_lib = true
   deps = [
     ":sdk",
   ]
 }
+
+# "Forwarding" target, to make it easy to put gtest wherever you want.
+# TODO(vtl): Maybe this should be parametrized and made into
+# //build/module_args/mojo.gni (but maybe that's overkill).
+group("gtest") {
+  testonly = true
+  public_deps = [
+    "//testing/gtest",
+  ]
+}