Replaced is_null() with is_valid in SurfaceId and related classes.

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"
 
 namespace cc {
 
 SurfaceIdAllocator::SurfaceIdAllocator() : next_id_(1u) {}
 
 SurfaceIdAllocator::~SurfaceIdAllocator() {
 }
 
 LocalFrameId SurfaceIdAllocator::GenerateId() {
   uint64_t nonce = base::RandUint64();
+  // Since 0 nonce are treated as uninitialized and thus invalid, we need to
+  // keep generating until there is a non-zero nonce
+  while (nonce == 0)
+    nonce = base::RandUint64();
   LocalFrameId id(next_id_, nonce);
   next_id_++;
   return id;
 }
 
 }  // namespace cc