allow GMs to animate

BUG=skia:

Review URL: https://codereview.chromium.org/888283002
diff --git a/samplecode/SampleArc.cpp b/samplecode/SampleArc.cpp
index df452ce..9f922ae 100644
--- a/samplecode/SampleArc.cpp
+++ b/samplecode/SampleArc.cpp
@@ -195,9 +195,13 @@
     }
 
     void onDrawContent(SkCanvas* canvas) SK_OVERRIDE {
-        fAnimatingDrawable->setSweep(SampleCode::GetAnimScalar(360/24, 360));
         canvas->EXPERIMENTAL_drawDrawable(fRootDrawable);
-        this->inval(NULL);
+    }
+
+    bool onAnimatePulse(SkMSec curr, SkMSec prev) SK_OVERRIDE {
+        SkScalar angle = SkDoubleToScalar(fmod(curr * 0.36 / 24, 360));
+        fAnimatingDrawable->setSweep(angle);
+        return true;
     }
 
     SkView::Click* onFindClickHandler(SkScalar x, SkScalar y, unsigned modi) SK_OVERRIDE {