Optionally clear PlatformCanvas instances.

content/browser/renderer_host/backing_store_mac.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 <Cocoa/Cocoa.h>
 
 #include "content/browser/renderer_host/backing_store_mac.h"
 
 #include "base/logging.h"
 #include "base/mac/mac_util.h"
@@ skipping 91 matching lines @@
       // The layer hasn't been created yet, so draw into the cache bitmap.
       gfx::Rect paint_rect = copy_rect;
       paint_rect.set_y(size().height() - copy_rect.bottom());
       CGContextDrawImage(cg_bitmap_, paint_rect.ToCGRect(), image);
     }
   }
 }
 
 bool BackingStoreMac::CopyFromBackingStore(const gfx::Rect& rect,
                                            skia::PlatformCanvas* output) {
-  if (!output->initialize(rect.width(), rect.height(), true))
+  if (!output->initialize(rect.width(), rect.height(),
+                          skia::PlatformDevice::FLAGS_OPAQUE))
     return false;
 
   skia::ScopedPlatformPaint scoped_platform_paint(output);
   CGContextRef temp_context = scoped_platform_paint.GetPlatformSurface();
   CGContextSaveGState(temp_context);
   CGContextTranslateCTM(temp_context, 0.0, size().height());
   CGContextScaleCTM(temp_context, 1.0, -1.0);
   CGContextDrawLayerAtPoint(temp_context, CGPointMake(rect.x(), rect.y()),
                             cg_layer());
   CGContextRestoreGState(temp_context);
@@ skipping 100 matching lines @@
   CGContextRef context = CGBitmapContextCreate(NULL,
                                                size().width(), size().height(),
                                                8, size().width() * 4,
                                                base::mac::GetSystemColorSpace(),
                                                kCGImageAlphaPremultipliedFirst |
                                                    kCGBitmapByteOrder32Host);
   DCHECK(context);
 
   return context;
 }