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

webkit/tools/test_shell/webwidget_host_gtk.cc

Index: webkit/tools/test_shell/webwidget_host_gtk.cc
===================================================================
--- webkit/tools/test_shell/webwidget_host_gtk.cc	(revision 167659)
+++ webkit/tools/test_shell/webwidget_host_gtk.cc	(working copy)
@@ -357,7 +357,8 @@
   if (!canvas_.get()) {
     ResetScrollRect();
     paint_rect_ = client_rect;
-    canvas_.reset(new skia::PlatformCanvas(width, height, true));
+    canvas_.reset(skia::CreatePlatformCanvas(width, height, true, 0,
+                                             skia::RETURN_NULL_ON_FAILURE));
     if (!canvas_.get()) {
       // memory allocation failed, we can't paint.
       LOG(ERROR) << "Failed to allocate memory for " << width << "x" << height;