Initialize bitmap to (0, 0, 0, 0) for CanvasPaintT<T> instances.

chrome/browser/ui/cocoa/extensions/browser_action_button.mm

 // 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.
 
 #import "chrome/browser/ui/cocoa/extensions/browser_action_button.h"
 
 #include <algorithm>
 #include <cmath>
 
 #include "base/logging.h"
@@ skipping 300 matching lines @@
 
 @end
 
 @implementation BrowserActionCell
 
 @synthesize tabId = tabId_;
 @synthesize extensionAction = extensionAction_;
 
 - (void)drawBadgeWithinFrame:(NSRect)frame {
   gfx::CanvasSkiaPaint canvas(frame, false);
-  canvas.sk_canvas()->clear(SkColorSetARGB(0, 0, 0, 0));
   canvas.set_composite_alpha(true);
   gfx::Rect boundingRect(NSRectToCGRect(frame));
   extensionAction_->PaintBadge(&canvas, boundingRect, tabId_);
 }
 
 - (void)drawInteriorWithFrame:(NSRect)cellFrame inView:(NSView*)controlView {
   gfx::ScopedNSGraphicsContextSaveGState scopedGState;
   [super drawInteriorWithFrame:cellFrame inView:controlView];
   cellFrame.origin.y += kBrowserActionBadgeOriginYOffset;
   [self drawBadgeWithinFrame:cellFrame];
 }
 
 @end