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

skia/ext/platform_canvas_skia.cc

Index: skia/ext/platform_canvas_skia.cc
===================================================================
--- skia/ext/platform_canvas_skia.cc	(revision 165342)
+++ skia/ext/platform_canvas_skia.cc	(working copy)
@@ -13,7 +13,7 @@
 // http://code.google.com/p/chromium/issues/detail?id=119555
 
 namespace skia {
-
+#if 0
 PlatformCanvas::PlatformCanvas(int width, int height, bool is_opaque) {
   TRACE_EVENT2("skia", "PlatformCanvas::PlatformCanvas",
                "width", width, "height", height);
@@ -109,5 +109,5 @@
       SkBitmap::kARGB_8888_Config, width, height, is_opaque));
 }
 #endif
-
+#endif
 }  // namespace skia