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

skia/ext/platform_canvas_mac.cc

Index: skia/ext/platform_canvas_mac.cc
===================================================================
--- skia/ext/platform_canvas_mac.cc	(revision 165342)
+++ skia/ext/platform_canvas_mac.cc	(working copy)
@@ -9,7 +9,7 @@
 #include "third_party/skia/include/core/SkTypes.h"
 
 namespace skia {
-
+#if 0
 PlatformCanvas::PlatformCanvas(int width, int height, bool is_opaque) {
   TRACE_EVENT2("skia", "PlatformCanvas::PlatformCanvas",
                "width", width, "height", height);
@@ -52,5 +52,5 @@
   return initializeWithDevice(BitmapPlatformDevice::Create(
       context, width, height, is_opaque));
 }
-
+#endif
 }  // namespace skia