Reenable running samplerstress GM in pipe mode.

It was disabled because samplerstress did was not
registered for flattening. Now that it is, do not
skip it when running GM in pipe mode.
Review URL: https://codereview.appspot.com/6305057

git-svn-id: http://skia.googlecode.com/svn/trunk@4196 2bbb7eff-a529-9590-31e7-b0007b416f81

gm/samplerstress.cpp

@@ -36,12 +36,6 @@ protected:
         return make_isize(640, 480);
     }
 
-    virtual uint32_t onGetFlags() const SK_OVERRIDE {
-        // Skip Pipe playback since we use a custom MaskFilter that will not be
-        // flattened correctly
-        return this->INHERITED::onGetFlags() | GM::kSkipPipe_Flag;
-    }
-
     /**
      * Create a red & green stripes on black texture
      */