add SkAnimTimer, SPACE = pause/resume, ESP = stop

BUG=skia:

Review URL: https://codereview.chromium.org/894083003
diff --git a/gm/addarc.cpp b/gm/addarc.cpp
index 1386cdd..e6fad25 100644
--- a/gm/addarc.cpp
+++ b/gm/addarc.cpp
@@ -6,6 +6,7 @@
  */
 
 #include "gm.h"
+#include "SkAnimTimer.h"
 #include "SkCanvas.h"
 #include "SkRandom.h"
 
@@ -49,8 +50,8 @@
         }
     }
 
-    bool onAnimatePulse(SkMSec curr, SkMSec prev) SK_OVERRIDE {
-        fRotate = SkDoubleToScalar(fmod(curr * 0.001, 360));
+    bool onAnimate(const SkAnimTimer& timer) SK_OVERRIDE {
+        fRotate = timer.scaled(1, 360);
         return true;
     }