Add auto purging for SkPicture-related Ganesh resources (esp. layers)

src/core/SkDevice.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDevice.h"
 #include "SkMetaData.h"
 
@@ skipping 116 matching lines @@
 }
 
 void* SkBaseDevice::onAccessPixels(SkImageInfo* info, size_t* rowBytes) {
     return NULL;
 }
 
 void SkBaseDevice::EXPERIMENTAL_optimize(const SkPicture* picture) {
     // The base class doesn't perform any analysis but derived classes may
 }
 
-void SkBaseDevice::EXPERIMENTAL_purge(const SkPicture* picture) {
-    // Derived-classes may have data to purge but not the base class
-}
-
 bool SkBaseDevice::EXPERIMENTAL_drawPicture(SkCanvas* canvas, const SkPicture* picture) {
     // The base class doesn't perform any accelerated picture rendering
     return false;
 }