Share backing canvases between browser compositors.

content/browser/compositor/software_output_device_win.h

Index: content/browser/compositor/software_output_device_win.h
diff --git a/content/browser/compositor/software_output_device_win.h b/content/browser/compositor/software_output_device_win.h
index 7063f207b00cfa446e5979cad1671c91bb465c9a..c09ca0ffba42f26943d37803c34d0998268643bc 100644
--- a/content/browser/compositor/software_output_device_win.h
+++ b/content/browser/compositor/software_output_device_win.h
@@ -19,7 +19,6 @@ class Compositor;
 }
 
 namespace content {
-
 class SoftwareOutputDeviceWin : public cc::SoftwareOutputDevice {
  public:
   explicit SoftwareOutputDeviceWin(ui::Compositor* compositor);
@@ -30,10 +29,12 @@ class SoftwareOutputDeviceWin : public cc::SoftwareOutputDevice {
   SkCanvas* BeginPaint(const gfx::Rect& damage_rect) override;
   void EndPaint(cc::SoftwareFrameData* frame_data) override;
 
+  gfx::Size viewport_pixel_size() const { return viewport_pixel_size_; }
+  void ReleaseContents();
+
  private:
   HWND hwnd_;
-  BITMAPINFO bitmap_info_;
-  scoped_ptr<gfx::Canvas> contents_;
+  scoped_ptr<SkCanvas> contents_;
   bool is_hwnd_composited_;
 
   DISALLOW_COPY_AND_ASSIGN(SoftwareOutputDeviceWin);