Skip tiling on bitmapscroll:

- off by exactly one pixel when using DirectWrite
    - also at least one pixel off on ChromeOS

BUG=skia:
R=bungeman@google.com

Review URL: https://codereview.chromium.org/394243002

gm/bitmapscroll.cpp

@@ -63,6 +63,10 @@ protected:
         return SkString("bitmapscroll");
     }
 
+    virtual uint32_t onGetFlags() const SK_OVERRIDE {
+        return kSkipTiled_Flag;
+    }
+
     virtual SkISize onISize() {
       return SkISize::Make(800, 600);
     }