Web Animations: Timeline should not advance during task execution

Source/core/animation/AnimationClock.cpp

 /*
  * Copyright (c) 2014, 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 13 matching lines @@
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "config.h"
 #include "core/animation/AnimationClock.h"
 
+#include "wtf/CurrentTime.h"
+#include <math.h>
+
+namespace {
+
+// FIXME: This is an approximation of time between frames, used when
+// ticking the animation clock outside of animation frame callbacks.
+// Ideally this would be generated by the compositor.
+const double approximateFrameTime = 1 / 60.0;
+
+}
+
 namespace WebCore {
 
+unsigned AnimationClock::s_currentTask = 0;
+
 void AnimationClock::updateTime(double time)
 {
     if (time > m_time)
         m_time = time;
-    m_frozen = true;
+    m_currentTask = s_currentTask;
 }
 
 double AnimationClock::currentTime()
 {
-    if (!m_frozen) {
-        double newTime = m_monotonicallyIncreasingTime();
-        if (newTime >= m_time + minTimeBeforeUnsynchronizedAnimationClockTick)
-            m_time = newTime;
+    if (m_currentTask != s_currentTask) {
+        const double currentTime = m_monotonicallyIncreasingTime();
+        if (m_time < currentTime) {
+            // Advance to the first estimated frame after the current time.
+            const double frameShift = fmod(currentTime - m_time, approximateFrameTime);
+            const double newTime = currentTime + (approximateFrameTime - frameShift);
+            ASSERT(newTime >= currentTime);
+            ASSERT(newTime <= currentTime + approximateFrameTime);
+            updateTime(newTime);
+        } else {
+            m_currentTask = s_currentTask;
+        }
     }
     return m_time;
 }
 
+void AnimationClock::resetTimeForTesting()
+{
+    m_time = 0;
+    m_currentTask = 0;
+    s_currentTask = 0;
 }
+
+}