Implement SkAshmemDiscardableMemory

tests/CachedDecodingPixelRefTest.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 "SkBitmap.h"
 #include "SkCanvas.h"
 #include "SkData.h"
@@ skipping 282 matching lines @@
         SkNEW_ARGS(SkDiscardableMemoryPool, (1, NULL)));
     REPORTER_ASSERT(reporter, 0 == pool->getRAMUsed());
     CheckPixelRef(TestImageGenerator::kFailGetPixels_TestType,
                   reporter, kSkDiscardable_PixelRefType, pool);
     REPORTER_ASSERT(reporter, 0 == pool->getRAMUsed());
     CheckPixelRef(TestImageGenerator::kSucceedGetPixels_TestType,
                   reporter, kSkDiscardable_PixelRefType, pool);
     REPORTER_ASSERT(reporter, 0 == pool->getRAMUsed());
 
     SkDiscardableMemoryPool* globalPool = SkGetGlobalDiscardableMemoryPool();
+    // Only acts differently from NULL on a platform that has a
+    // default discardable memory implementation that differs from the
+    // global DM pool.
     CheckPixelRef(TestImageGenerator::kFailGetPixels_TestType,
                   reporter, kSkDiscardable_PixelRefType, globalPool);
     CheckPixelRef(TestImageGenerator::kSucceedGetPixels_TestType,
                   reporter, kSkDiscardable_PixelRefType, globalPool);
-
-    // TODO(halcanary): When ashmem-backed SkDiscardableMemory lands,
-    // test that here (on platforms where it is availible).
 }
 ////////////////////////////////////////////////////////////////////////////////