BitmapContentLayerUpdater shouldn't reallocate bitmap smaller.

ui/compositor/layer_unittest.cc

 // Copyright (c) 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 "base/basictypes.h"
 #include "base/bind.h"
 #include "base/compiler_specific.h"
 #include "base/file_util.h"
 #include "base/files/file_path.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 158 matching lines @@
   std::string ToScaleString() const {
     return base::StringPrintf("%.1f %.1f", scale_x_, scale_y_);
   }
 
   float device_scale_factor() const {
     return device_scale_factor_;
   }
 
   // Overridden from LayerDelegate:
   virtual void OnPaintLayer(gfx::Canvas* canvas) OVERRIDE {
-    gfx::ImageSkiaRep contents = canvas->ExtractImageRep();
-    paint_size_ = gfx::Size(contents.pixel_width(), contents.pixel_height());
+    SkIRect clip_bounds;
+    canvas->sk_canvas()->getClipDeviceBounds(&clip_bounds);
+    paint_size_ = gfx::Size(clip_bounds.width(), clip_bounds.height());
     canvas->FillRect(gfx::Rect(paint_size_), colors_[color_index_]);
     color_index_ = (color_index_ + 1) % static_cast<int>(colors_.size());
     const SkMatrix& matrix = canvas->sk_canvas()->getTotalMatrix();
     scale_x_ = matrix.getScaleX();
     scale_y_ = matrix.getScaleY();
   }
 
   virtual void OnDeviceScaleFactorChanged(float device_scale_factor) OVERRIDE {
     device_scale_factor_ = device_scale_factor;
   }
@@ skipping 1214 matching lines @@
   l1->SetOpacity(0.5f);
 
   // Change l1's cc::Layer.
   l1->SwitchCCLayerForTest();
 
   // Ensure that the opacity animation completed.
   EXPECT_FLOAT_EQ(l1->opacity(), 0.5f);
 }
 
 }  // namespace ui