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

ppapi/thunk/ppb_image_data_api.h

 // Copyright (c) 2012 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 PPAPI_THUNK_PPB_IMAGE_DATA_API_H_
 #define PPAPI_THUNK_PPB_IMAGE_DATA_API_H_
 
 #include "ppapi/c/pp_bool.h"
 #include "ppapi/c/ppb_image_data.h"
 
-namespace skia {
-class PlatformCanvas;
-}
-
 class SkCanvas;
 
 namespace ppapi {
 namespace thunk {
 
 class PPB_ImageData_API {
  public:
   virtual ~PPB_ImageData_API() {}
 
   virtual PP_Bool Describe(PP_ImageDataDesc* desc) = 0;
   virtual void* Map() = 0;
   virtual void Unmap() = 0;
 
   // Trusted inteface.
   virtual int32_t GetSharedMemory(int* handle, uint32_t* byte_count) = 0;
 
   // Get the platform-specific canvas that backs this ImageData, if there is
   // one.
   // The canvas will be NULL:
   //   * If the image is not mapped.
   //   * Within untrusted code (which does not have skia).
   //   * If the ImageData is not backed by a platform-specific image buffer.
   //     This will be the case for ImageDatas created for use in NaCl.
   // For this last reason, you should prefer GetCanvas any time you don't need
   // a platform-specific canvas (e.g., for use with platform-specific APIs).
   // Anything that relies on having a PlatformCanvas will not work for ImageDat
   // objects created from NaCl.
-  virtual skia::PlatformCanvas* GetPlatformCanvas() = 0;
+  virtual SkCanvas* GetPlatformCanvas() = 0;
 
   // Get the canvas that backs this ImageData, if there is one.
   // The canvas will be NULL:
   //   * If the image is not mapped.
   //   * Within untrusted code (which does not have skia).
   virtual SkCanvas* GetCanvas() = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_PPB_IMAGE_DATA_API_H_