Introduce PlatformBitmap, which is a minimal helper class that wraps an SkBitmap

content/port/browser/render_widget_host_view_port.h

Index: content/port/browser/render_widget_host_view_port.h
===================================================================
--- content/port/browser/render_widget_host_view_port.h	(revision 161107)
+++ content/port/browser/render_widget_host_view_port.h	(working copy)
@@ -35,6 +35,10 @@
 }
 #endif
 
+namespace skia {
+class PlatformBitmap;
+};
+
 namespace content {
 class BackingStore;
 class SmoothScrollGesture;
@@ -161,7 +165,7 @@
       const gfx::Rect& src_subrect,
       const gfx::Size& dst_size,
       const base::Callback<void(bool)>& callback,
-      skia::PlatformCanvas* output) = 0;
+      skia::PlatformBitmap* output) = 0;
 
   // Called when accelerated compositing state changes.
   virtual void OnAcceleratedCompositingStateChange() = 0;