Add an option on SkImageDecoder to skip writing 0s.

src/images/SkImageRef_ashmem.cpp

Index: src/images/SkImageRef_ashmem.cpp
diff --git a/src/images/SkImageRef_ashmem.cpp b/src/images/SkImageRef_ashmem.cpp
index a4058ff6d77b0d42581194c9bdfa89989788655a..913792fa4fb9a5cd4e54483087541fed733d1ffe 100644
--- a/src/images/SkImageRef_ashmem.cpp
+++ b/src/images/SkImageRef_ashmem.cpp
@@ -134,6 +134,9 @@ bool SkImageRef_ashmem::onDecode(SkImageDecoder* codec, SkStream* stream,
         return this->INHERITED::onDecode(codec, stream, bitmap, config, mode);
     }
 
+    // Ashmem memory is guaranteed to be initialized to 0.
+    codec->setSkipWritingZeroes(true);
+
     AshmemAllocator alloc(&fRec, this->getURI());
 
     codec->setAllocator(&alloc);