Optionally clear PlatformCanvas instances.

webkit/tools/test_shell/mac/webwidget_host.mm

 // Copyright (c) 2011 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 "webkit/tools/test_shell/webwidget_host.h"
 
 #include "base/logging.h"
 #include "skia/ext/platform_canvas.h"
@@ skipping 157 matching lines @@
   gfx::Rect client_rect(NSRectToCGRect([view_ frame]));
   NSGraphicsContext* view_context = [NSGraphicsContext currentContext];
   CGContextRef context = static_cast<CGContextRef>([view_context graphicsPort]);
 
   // Allocate a canvas if necessary
   if (!canvas_.get()) {
     ResetScrollRect();
     paint_rect_ = client_rect;
     canvas_.reset(new skia::PlatformCanvas(
         paint_rect_.width(), paint_rect_.height(), true));
+    canvas_->drawARGB(0, 0, 0, 0);
   }
 
   // make sure webkit draws into our bitmap, not the window
   CGContextRef bitmap_context =
       skia::GetBitmapContext(skia::GetTopDevice(*canvas_));
   [NSGraphicsContext setCurrentContext:
       [NSGraphicsContext graphicsContextWithGraphicsPort:bitmap_context
                                                  flipped:YES]];
 
   webwidget_->animate(0.0);
@@ skipping 89 matching lines @@
 void WebWidgetHost::PaintRect(const gfx::Rect& rect) {
 #ifndef NDEBUG
   DCHECK(!painting_);
 #endif
   DCHECK(canvas_.get());
 
   set_painting(true);
   webwidget_->paint(webkit_glue::ToWebCanvas(canvas_.get()), rect);
   set_painting(false);
 }