Implement SkAshmemDiscardableMemory

src/lazy/SkDiscardablePixelRef.cpp

Index: src/lazy/SkDiscardablePixelRef.cpp
diff --git a/src/lazy/SkDiscardablePixelRef.cpp b/src/lazy/SkDiscardablePixelRef.cpp
index b6e1b1004ed7096fc180be5945aa44e5dfab1c1c..026186a114de6d94c8a8a7abdcb9decab6acf0e0 100644
--- a/src/lazy/SkDiscardablePixelRef.cpp
+++ b/src/lazy/SkDiscardablePixelRef.cpp
@@ -31,6 +31,7 @@ void* SkDiscardablePixelRef::onLockPixels(SkColorTable**) {
         if (fDiscardableMemory->lock()) {
             return fDiscardableMemory->data();
         }
+        SkDELETE(fDiscardableMemory);
         fDiscardableMemory = NULL;
     }
     fDiscardableMemory = SkDiscardableMemory::Create(fSize);