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

ppapi/shared_impl/private/ppb_browser_font_trusted_shared.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/shared_impl/private/ppb_browser_font_trusted_shared.h"
 
 #include "base/string_util.h"
 #include "base/utf_string_conversions.h"
 #include "ppapi/c/dev/ppb_font_dev.h"
 #include "ppapi/shared_impl/ppapi_preferences.h"
@@ skipping 303 matching lines @@
     const PP_Rect* clip,
     PP_Bool image_data_is_opaque) {
   PP_Bool result = PP_FALSE;
   // Get and map the image data we're painting to.
   EnterResourceNoLock<PPB_ImageData_API> enter(image_data, true);
   if (enter.failed())
     return result;
 
   PPB_ImageData_API* image = static_cast<PPB_ImageData_API*>(
       enter.object());
-  skia::PlatformCanvas* canvas = image->GetPlatformCanvas();
+  SkCanvas* canvas = image->GetPlatformCanvas();
   bool needs_unmapping = false;
   if (!canvas) {
     needs_unmapping = true;
     image->Map();
     canvas = image->GetPlatformCanvas();
     if (!canvas)
       return result;  // Failure mapping.
   }
 
   DrawTextToCanvas(canvas, *text, position, color, clip, image_data_is_opaque);
@@ skipping 57 matching lines @@
     } else {
       // Character is past this run, account for the pixels and continue
       // looking.
       cur_pixel_offset += font_->calculateWidth(run);
     }
   }
   return -1;  // Requested a char beyond the end.
 }
 
 void PPB_BrowserFont_Trusted_Shared::DrawTextToCanvas(
-    skia::PlatformCanvas* destination,
+    SkCanvas* destination,
     const PP_BrowserFont_Trusted_TextRun& text,
     const PP_Point* position,
     uint32_t color,
     const PP_Rect* clip,
     PP_Bool image_data_is_opaque) {
   // Convert position and clip.
   WebFloatPoint web_position(static_cast<float>(position->x),
                              static_cast<float>(position->y));
   WebRect web_clip;
   if (!clip) {
@@ skipping 18 matching lines @@
 
     // Advance to the next run. Note that we avoid doing this for the last run
     // since it's unnecessary, measuring text is slow, and most of the time
     // there will be only one run anyway.
     if (i != runs.num_runs() - 1)
       web_position.x += font_->calculateWidth(run);
   }
 }
 
 }  // namespace ppapi