More accurate render time and continuous fresh

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2129613002

Review-Url: https://codereview.chromium.org/2129613002
diff --git a/tools/viewer/Viewer.h b/tools/viewer/Viewer.h
index e140e1b..1ed135d 100644
--- a/tools/viewer/Viewer.h
+++ b/tools/viewer/Viewer.h
@@ -24,7 +24,7 @@
     ~Viewer() override;
 
     void onPaint(SkCanvas* canvas);
-    void onIdle(double ms) override;
+    void onIdle() override;
     bool onTouch(intptr_t owner, sk_app::Window::InputState state, float x, float y);
     void onUIStateChanged(const SkString& stateName, const SkString& stateValue);
 
@@ -52,6 +52,7 @@
     int                    fCurrentSlide;
 
     bool                   fDisplayStats;
+    bool                   fRefresh; // whether to continuously refresh for measuring render time
 
     // whether to split the screen and draw two copies of the slide, one with sRGB and one without
     bool                   fSplitScreen;