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

ppapi/proxy/ppb_image_data_proxy.cc

 // 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.
 
 #include "ppapi/proxy/ppb_image_data_proxy.h"
 
 #include <string.h>  // For memcpy
 
 #include <map>
 #include <vector>
@@ skipping 368 matching lines @@
 #endif
 }
 
 int32_t ImageData::GetSharedMemory(int* /* handle */,
                                    uint32_t* /* byte_count */) {
   // Not supported in the proxy (this method is for actually implementing the
   // proxy in the host).
   return PP_ERROR_NOACCESS;
 }
 
-skia::PlatformCanvas* ImageData::GetPlatformCanvas() {
+SkCanvas* ImageData::GetPlatformCanvas() {
 #if defined(OS_NACL)
   return NULL;  // No canvas in NaCl.
 #else
   return mapped_canvas_.get();
 #endif
 }
 
 SkCanvas* ImageData::GetCanvas() {
 #if defined(OS_NACL)
   return NULL;  // No canvas in NaCl.
@@ skipping 224 matching lines @@
   // still cached in our process, the proxy still holds a reference so we can
   // remove the one the renderer just sent is. If the proxy no longer holds a
   // reference, we released everything and we should also release the one the
   // renderer just sent us.
   dispatcher()->Send(new PpapiHostMsg_PPBCore_ReleaseResource(
       API_ID_PPB_CORE, old_image_data));
 }
 
 }  // namespace proxy
 }  // namespace ppapi