[Android] Clear backing SkBitmap before rendering the overscroll effect

content/browser/android/system_ui_resource_manager_impl.cc

 // Copyright 2014 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/browser/android/system_ui_resource_manager_impl.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/debug/trace_event.h"
 #include "base/location.h"
@@ skipping 27 matching lines @@
   const float height = arc_width - y;
   gfx::Size bounds(arc_width, height);
   SkRect arc_rect = SkRect::MakeXYWH(
       -arc_width / 2.f, -arc_width - y, arc_width * 2.f, arc_width * 2.f);
   SkBitmap glow_bitmap;
   if (!glow_bitmap.allocPixels(
           SkImageInfo::MakeA8(bounds.width(), bounds.height()))) {
     LOG(FATAL) << " Failed to allocate bitmap of size " << bounds.width() << "x"
                << bounds.height();
   }
+  glow_bitmap.eraseColor(SK_ColorTRANSPARENT);
 
   SkCanvas canvas(glow_bitmap);
   canvas.clipRect(SkRect::MakeXYWH(0, 0, bounds.width(), bounds.height()));
   canvas.drawArc(arc_rect, 45, 90, true, paint);
   return glow_bitmap;
 }
 
 void LoadBitmap(ui::SystemUIResourceManager::ResourceType type,
                 SkBitmap* bitmap_holder,
                 const gfx::Size& screen_size) {
@@ skipping 115 matching lines @@
 }
 
 void SystemUIResourceManagerImpl::OnFinishedLoadBitmap(
     ResourceType type,
     SkBitmap* bitmap_holder) {
   DCHECK(bitmap_holder);
   GetEntry(type)->SetBitmap(*bitmap_holder);
 }
 
 }  // namespace content