Simplify platform_canvas.h by recognizing that PlatformCanvas does not actually extend

content/renderer/render_process.h

Index: content/renderer/render_process.h
===================================================================
--- content/renderer/render_process.h	(revision 167659)
+++ content/renderer/render_process.h	(working copy)
@@ -14,10 +14,6 @@
 class Rect;
 }
 
-namespace skia {
-class PlatformCanvas;
-}
-
 namespace content {
 
 // A abstract interface representing the renderer end of the browser<->renderer
@@ -39,8 +35,8 @@
   //
   // When no longer needed, you should pass the TransportDIB to
   // ReleaseTransportDIB so that it can be recycled.
-  virtual skia::PlatformCanvas* GetDrawingCanvas(TransportDIB** memory,
-                                                 const gfx::Rect& rect) = 0;
+  virtual SkCanvas* GetDrawingCanvas(TransportDIB** memory,
+                                     const gfx::Rect& rect) = 0;
 
   // Frees shared memory allocated by AllocSharedMemory.  You should only use
   // this function to free the SharedMemory object.