Make cc::SurfaceId unguessable

cc/surfaces/surface_id_allocator.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_id_allocator.h"
 
 #include <stdint.h>
 
+#include "base/rand_util.h"
 #include "cc/surfaces/surface_manager.h"
 
 namespace cc {
 
 SurfaceIdAllocator::SurfaceIdAllocator(uint32_t id_namespace)
     : id_namespace_(id_namespace), next_id_(1u), manager_(nullptr) {
 }
 
 void SurfaceIdAllocator::RegisterSurfaceIdNamespace(SurfaceManager* manager) {
   DCHECK(!manager_);
   manager_ = manager;
   manager_->RegisterSurfaceIdNamespace(id_namespace_);
 }
 
 SurfaceIdAllocator::~SurfaceIdAllocator() {
   if (manager_)
     manager_->InvalidateSurfaceIdNamespace(id_namespace_);
 }
 
 SurfaceId SurfaceIdAllocator::GenerateId() {
-  SurfaceId id(static_cast<uint64_t>(id_namespace_) << 32 | next_id_);
+  uint64_t nonce = base::RandUint64();
+  SurfaceId id(id_namespace_, next_id_, nonce);
   next_id_++;
   return id;
 }
 
-// static
-uint32_t SurfaceIdAllocator::NamespaceForId(SurfaceId id) {
-  return id.id >> 32;
-}
-
 }  // namespace cc