Add a detachAsStream to SkDynamicMemoryWStream.

gm/gmmain.cpp

Index: gm/gmmain.cpp
===================================================================
--- gm/gmmain.cpp	(revision 10125)
+++ gm/gmmain.cpp	(working copy)
@@ -411,10 +411,10 @@
     }
 
     static bool write_document(const SkString& path,
-                               const SkDynamicMemoryWStream& document) {
+                               SkDynamicMemoryWStream& document) {
         SkFILEWStream stream(path.c_str());
-        SkAutoDataUnref data(document.copyToData());
-        return stream.write(data->data(), data->size());
+        SkAutoTUnref<SkStreamAsset> data(document.detatchAsStream());
+        return stream.writeStream(data.get(), data->getLength());
     }
 
     /**