cc: Return a NullCanvas instead of NULL in context lost situations.

cc/resources/resource_provider_unittest.cc

 // Copyright 2012 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/resources/resource_provider.h"
 
 #include <algorithm>
 #include <map>
 #include <set>
 
@@ skipping 3305 matching lines @@
 
   EXPECT_CALL(*context, NextTextureId()).WillRepeatedly(Return(texture_id));
 
   id = resource_provider->CreateResource(
       size, GL_CLAMP_TO_EDGE, ResourceProvider::TextureHintImmutable, format);
   context->loseContextCHROMIUM(GL_GUILTY_CONTEXT_RESET_ARB,
                                GL_INNOCENT_CONTEXT_RESET_ARB);
 
   RasterBuffer* raster_buffer = resource_provider->AcquirePixelRasterBuffer(id);
   skia::RefPtr<SkCanvas> canvas = raster_buffer->AcquireSkCanvas();
-  EXPECT_TRUE(canvas.get() == NULL);
+  EXPECT_TRUE(canvas);
   raster_buffer->ReleaseSkCanvas(canvas);
-  canvas.clear();
   resource_provider->ReleasePixelRasterBuffer(id);
   Mock::VerifyAndClearExpectations(context);
 }
 
 TEST_P(ResourceProviderTest, Image_GLTexture) {
   // Only for GL textures.
   if (GetParam() != ResourceProvider::GLTexture)
     return;
   scoped_ptr<AllocationTrackingContext3D> context_owned(
       new StrictMock<AllocationTrackingContext3D>);
@@ skipping 479 matching lines @@
     resource_provider->AllocateForTesting(id);
     Mock::VerifyAndClearExpectations(context);
 
     DCHECK_EQ(10u, context->PeekTextureId());
     resource_provider->DeleteResource(id);
   }
 }
 
 }  // namespace
 }  // namespace cc