Fix size of vertices GM to show all content

Change-Id: Ib0c567da03fe9373cdfeb23291c1f448571668b3
Reviewed-on: https://skia-review.googlesource.com/8317
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>

gm/vertices.cpp

@@ -122,7 +122,7 @@ protected:
     }
 
     SkISize onISize() override {
-        return SkISize::Make(600, 600);
+        return SkISize::Make(975, 1175);
     }
 
     void onDraw(SkCanvas* canvas) override {