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

content/browser/renderer_host/backing_store.h

Index: content/browser/renderer_host/backing_store.h
===================================================================
--- content/browser/renderer_host/backing_store.h	(revision 161107)
+++ content/browser/renderer_host/backing_store.h	(working copy)
@@ -20,7 +20,7 @@
 }
 
 namespace skia {
-class PlatformCanvas;
+class PlatformBitmap;
 }
 
 namespace content {
@@ -66,7 +66,7 @@
   // will call initialize() with the correct size. The return value indicates
   // success.
   virtual bool CopyFromBackingStore(const gfx::Rect& rect,
-                                    skia::PlatformCanvas* output) = 0;
+                                    skia::PlatformBitmap* output) = 0;
 
   // Scrolls the contents of clip_rect in the backing store by dx or dy (but dx
   // and dy cannot both be non-zero).