[Merge to M52]Fill the canvas with black background before painting wallpaper.

ash/desktop_background/desktop_background_view.cc

 // 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.
 
 #include "ash/desktop_background/desktop_background_view.h"
 
 #include <limits>
 
 #include "ash/ash_export.h"
 #include "ash/desktop_background/desktop_background_controller.h"
@@ skipping 120 matching lines @@
 void DesktopBackgroundView::OnPaint(gfx::Canvas* canvas) {
   // Scale the image while maintaining the aspect ratio, cropping as
   // necessary to fill the background. Ideally the image should be larger
   // than the largest display supported, if not we will scale and center it if
   // the layout is WALLPAPER_LAYOUT_CENTER_CROPPED.
   DesktopBackgroundController* controller =
       Shell::GetInstance()->desktop_background_controller();
   gfx::ImageSkia wallpaper = controller->GetWallpaper();
   WallpaperLayout wallpaper_layout = controller->GetWallpaperLayout();
 
-  if (wallpaper.isNull()) {
-    canvas->FillRect(GetLocalBounds(), SK_ColorBLACK);
+  // Wallpapers with png format could be partially transparent.
+  // Fill the canvas with black background to make it opaque
+  // before painting wallpaper
+  canvas->FillRect(GetLocalBounds(), SK_ColorBLACK);
+
+  if (wallpaper.isNull())
     return;
-  }
 
   if (wallpaper_layout == WALLPAPER_LAYOUT_CENTER_CROPPED) {
     // The dimension with the smallest ratio must be cropped, the other one
     // is preserved. Both are set in gfx::Size cropped_size.
     double horizontal_ratio = static_cast<double>(width()) /
         static_cast<double>(wallpaper.width());
     double vertical_ratio = static_cast<double>(height()) /
         static_cast<double>(wallpaper.height());
 
     gfx::Size cropped_size;
@@ skipping 15 matching lines @@
         wallpaper_cropped_rect.width(), wallpaper_cropped_rect.height(),
         0, 0, width(), height(),
         true);
   } else if (wallpaper_layout == WALLPAPER_LAYOUT_TILE) {
     canvas->TileImageInt(wallpaper, 0, 0, width(), height());
   } else if (wallpaper_layout == WALLPAPER_LAYOUT_STRETCH) {
     // This is generally not recommended as it may show artifacts.
     canvas->DrawImageInt(wallpaper, 0, 0, wallpaper.width(),
         wallpaper.height(), 0, 0, width(), height(), true);
   } else {
-    // Fill with black to make sure that the entire area is opaque.
-    canvas->FillRect(GetLocalBounds(), SK_ColorBLACK);
     float image_scale = canvas->image_scale();
     gfx::Rect wallpaper_rect(0, 0, wallpaper.width() / image_scale,
                              wallpaper.height() / image_scale);
     // All other are simply centered, and not scaled (but may be clipped).
     canvas->DrawImageInt(
         wallpaper,
         0, 0, wallpaper.width(), wallpaper.height(),
         (width() - wallpaper_rect.width()) / 2,
         (height() - wallpaper_rect.height()) / 2,
         wallpaper_rect.width(),
@@ skipping 57 matching lines @@
     // Disable animation if transition to login screen from an empty background.
     ::wm::SetWindowVisibilityAnimationTransition(
         desktop_widget->GetNativeView(), ::wm::ANIMATE_NONE);
   }
 
   desktop_widget->SetBounds(params.parent->bounds());
   return desktop_widget;
 }
 
 }  // namespace ash