Add SkPurgeGlobalDiscardableMemoryPool, needed to replace SkImageRef

src/lazy/SkDiscardableMemoryPool.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDiscardableMemory.h"
 #include "SkDiscardableMemoryPool.h"
 #include "SkOnce.h"
@@ skipping 253 matching lines @@
     return SkNEW_ARGS(DiscardableMemoryPool, (size, mutex));
 }
 
 SkDiscardableMemoryPool* SkGetGlobalDiscardableMemoryPool() {
     SK_DECLARE_STATIC_ONCE(create_pool_once);
     SkOnce(&create_pool_once, create_global_pool, 0, &cleanup_global_pool);
     SkASSERT(NULL != gPool);
     return gPool;
 }
 
+// defined in SkImageGenerator.h
+void SkPurgeGlobalDiscardableMemoryPool() {
+    SkGetGlobalDiscardableMemoryPool()->dumpPool();
+}
 ////////////////////////////////////////////////////////////////////////////////