Make copySurface work for texture dsts, return a bool, & add unit test.

src/gpu/GrDrawTarget.h

Index: src/gpu/GrDrawTarget.h
diff --git a/src/gpu/GrDrawTarget.h b/src/gpu/GrDrawTarget.h
index 55c11da6674d5f334df6640062747d8f5c7ffdcd..a850efd842f1b9d20719323ec577f25217ed76da 100644
--- a/src/gpu/GrDrawTarget.h
+++ b/src/gpu/GrDrawTarget.h
@@ -144,7 +144,7 @@ public:
      * depending on the type of surface, configs, etc, and the backend-specific
      * limitations.
      */
-    void copySurface(GrSurface* dst,
+    bool copySurface(GrSurface* dst,
                      GrSurface* src,
                      const SkIRect& srcRect,
                      const SkIPoint& dstPoint);