Defer starting of animations until after compositing update

Source/core/animation/AnimationClock.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 16 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef AnimationClock_h
 #define AnimationClock_h
 
 #include "wtf/CurrentTime.h"
 #include "wtf/PassOwnPtr.h"
 
+namespace {

[abarth-chromium, 2014/02/20 17:52:00]

Please don't use anonymous namespaces in headers.  That makes the linker go
crazy.

[dstockwell, 2014/02/24 06:03:25]

Done.

+
+// FIXME: This value is used to suppress updates when time is required outside of a frame.
+// The purpose of allowing the clock to update during such periods is to allow animations
+// to have an appropriate start time and for getComputedStyle to attempt to catch-up to a
+// compositor animation. However a more accurate system might be to attempt to phase-lock
+// with the frame clock.
+const double minTimeBeforeUnsynchronizedTick = 0.005;
+
+}
+
 namespace WebCore {
 
 class AnimationClock {
 public:
     static PassOwnPtr<AnimationClock> create(WTF::TimeFunction monotonicallyIncreasingTime = WTF::monotonicallyIncreasingTime)
     {
         return adoptPtr(new AnimationClock(monotonicallyIncreasingTime));
     }
 
     void updateTime(double time)
     {
         if (time > m_time)
             m_time = time;
         m_frozen = true;
     }
 
     double currentTime()
     {
-        if (!m_frozen)
-            updateTime(m_monotonicallyIncreasingTime());
+        if (!m_frozen) {
+            double newTime = m_monotonicallyIncreasingTime();
+            if (newTime >= m_time + minTimeBeforeUnsynchronizedTick)
+                m_time = newTime;
+        }
         return m_time;
     }
 
     void unfreeze() { m_frozen = false; }
 
     void resetTimeForTesting() { m_time = 0; m_frozen = true; }
 
 private:
     AnimationClock(WTF::TimeFunction monotonicallyIncreasingTime)
         : m_monotonicallyIncreasingTime(monotonicallyIncreasingTime)
         , m_time(0)
         , m_frozen(false)
     {
     }
     WTF::TimeFunction m_monotonicallyIncreasingTime;
     double m_time;
     bool m_frozen;
 };
 
 } // namespace WebCore
 
 #endif // AnimationClock_h