Use a struct for cc::Surface ids for more type safety

cc/surfaces/surface_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 "cc/surfaces/surface.h"
 #include "cc/surfaces/surface_manager.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/gfx/size.h"
 
 namespace cc {
 namespace {
 
 TEST(SurfaceTest, SurfaceLifetime) {
   SurfaceManager manager;
 
-  int surface_id = 0;
+  SurfaceId surface_id;
   {
     Surface surface(&manager, NULL, gfx::Size(5, 5));
     surface_id = surface.surface_id();
-    EXPECT_GT(surface_id, 0);
-    EXPECT_EQ(&surface, manager.GetSurfaceForID(surface_id));
+    EXPECT_TRUE(!surface_id.is_null());
+    EXPECT_EQ(&surface, manager.GetSurfaceForId(surface_id));
   }
 
-  EXPECT_EQ(NULL, manager.GetSurfaceForID(surface_id));
+  EXPECT_EQ(NULL, manager.GetSurfaceForId(surface_id));
 }
 
 }  // namespace
 }  // namespace cc