Initialize bitmap to (0, 0, 0, 0) for CanvasPaintT<T> instances.

skia/ext/canvas_paint_x.h

 // 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.
 
 #ifndef SKIA_EXT_CANVAS_PAINT_WAYLAND_H_
 #define SKIA_EXT_CANVAS_PAINT_WAYLAND_H_
 #pragma once
 
 #include "base/logging.h"
 #include "skia/ext/canvas_paint_common.h"
 #include "skia/ext/platform_canvas.h"
 
 #include <cairo/cairo.h>
 
 namespace skia {
 
 // A class designed to translate skia painting into a region in a Wayland window
 // surface. On construction, it will set up a context for painting into, and on
 // destruction, it will commit it to the Wayland window surface.
+// Note: The created context is always inialized to (0, 0, 0, 0).
 template <class T>
 class CanvasPaintT : public T {
  public:
   // This constructor assumes the result is opaque.
   CanvasPaintT(cairo_surface_t* cairo_window_surface,
                cairo_rectangle_t* region)
       : context_(NULL),
         cairo_window_surface_(cairo_window_surface),
         region_(region),
         composite_alpha_(false) {
@@ skipping 51 matching lines @@
     return region_->width == 0 && region_->height == 0;
   }
 
  private:
   void init(bool opaque) {
     PlatformCanvas* canvas = GetPlatformCanvas(this);
     if (!canvas->initialize(region_->width, region_->height, opaque, NULL)) {
       // Cause a deliberate crash;
       CHECK(false);
     }
+    // No need to clear the canvas, because cairo automatically performs the
+    // clear.
 
     // Need to translate so that the dirty region appears at the origin of the
     // surface.
     canvas->translate(-SkDoubleToScalar(region_->x),
                       -SkDoubleToScalar(region_->y));
 
     context_ = BeginPlatformPaint(canvas);
   }
 
   cairo_t* context_;
   cairo_surface_t* cairo_window_surface_;
   cairo_rectangle_t* region_;
   // See description above setter.
   bool composite_alpha_;
 
   // Disallow copy and assign.
   CanvasPaintT(const CanvasPaintT&);
   CanvasPaintT& operator=(const CanvasPaintT&);
 };
 
 typedef CanvasPaintT<PlatformCanvas> PlatformCanvasPaint;
 
 }  // namespace skia
 
 #endif  // SKIA_EXT_CANVAS_PAINT_WAYLAND_H_