Add buffer helper to read directly into SkString to avoid extra copying

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 dc604651249c84915eb19ce8223305f110e185f1..a4058ff6d77b0d42581194c9bdfa89989788655a 100644
--- a/src/images/SkImageRef_ashmem.cpp
+++ b/src/images/SkImageRef_ashmem.cpp
@@ -223,9 +223,8 @@ SkImageRef_ashmem::SkImageRef_ashmem(SkFlattenableReadBuffer& buffer)
     fRec.fSize = 0;
     fRec.fPinned = false;
     fCT = NULL;
-    char* uri = buffer.readString();
-    if (uri) {
-        setURI(uri);
-        sk_free(uri);
-    }
+
+    SkString uri;
+    buffer.readString(&uri);
+    this->setURI(uri);
 }