Add loopSKP flag to nanobench

Adds a loopSKP flag that forces SKPBenches to draw with only 1 loop.

BUG=skia:

Review URL: https://codereview.chromium.org/1203193002
diff --git a/bench/SKPBench.h b/bench/SKPBench.h
index 3b645e9..e1c66bf 100644
--- a/bench/SKPBench.h
+++ b/bench/SKPBench.h
@@ -18,9 +18,13 @@
 class SKPBench : public Benchmark {
 public:
     SKPBench(const char* name, const SkPicture*, const SkIRect& devClip, SkScalar scale,
-             bool useMultiPictureDraw);
+             bool useMultiPictureDraw, bool doLooping);
     ~SKPBench() override;
 
+    int calculateLoops(int defaultLoops) const override {
+        return fDoLooping ? defaultLoops : 1;
+    }
+
 protected:
     const char* onGetName() override;
     const char* onGetUniqueName() override;
@@ -48,6 +52,8 @@
     SkTDArray<SkSurface*> fSurfaces;   // for MultiPictureDraw
     SkTDArray<SkIRect> fTileRects;     // for MultiPictureDraw
 
+    const bool fDoLooping;
+
     typedef Benchmark INHERITED;
 };