[Aura] Support transparent webkit.

content/renderer/render_widget.cc

 // 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.
 
 #include "content/renderer/render_widget.h"
 
 #include "base/command_line.h"
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 516 matching lines @@
   canvas->translate(static_cast<SkScalar>(-canvas_origin.x()),
                     static_cast<SkScalar>(-canvas_origin.y()));
 
   // If there is a custom background, tile it.
   if (!background_.empty()) {
     SkPaint paint;
     SkShader* shader = SkShader::CreateBitmapShader(background_,
                                                     SkShader::kRepeat_TileMode,
                                                     SkShader::kRepeat_TileMode);
     paint.setShader(shader)->unref();
-    paint.setXfermodeMode(SkXfermode::kSrcOver_Mode);
+
+    // Use kSrc_Mode to handle background_ transparency properly.
+    paint.setXfermodeMode(SkXfermode::kSrc_Mode);
+
+    // Canvas could contain multiple update rects. Clip to given rect so that
+    // we don't accidentally clear other update rects.
+    canvas->save();
+    SkRect clip;
+    clip.set(SkIntToScalar(rect.x()), SkIntToScalar(rect.y()),
+             SkIntToScalar(rect.right()), SkIntToScalar(rect.bottom()));
+    canvas->clipRect(clip);
     canvas->drawPaint(paint);
+    canvas->restore();
   }
 
   // First see if this rect is a plugin that can paint itself faster.
   TransportDIB* optimized_dib = NULL;
   gfx::Rect optimized_copy_rect, optimized_copy_location;
   webkit::ppapi::PluginInstance* optimized_instance =
       GetBitmapForOptimizedPluginPaint(rect, &optimized_dib,
                                        &optimized_copy_location,
                                        &optimized_copy_rect);
   if (optimized_instance) {
@@ skipping 879 matching lines @@
   }
 }
 
 bool RenderWidget::WillHandleMouseEvent(const WebKit::WebMouseEvent& event) {
   return false;
 }
 
 bool RenderWidget::WebWidgetHandlesCompositorScheduling() const {
   return false;
 }