Web Animations: Timeline should not advance during task execution

Source/core/animation/AnimationClock.cpp

Index: Source/core/animation/AnimationClock.cpp
diff --git a/Source/core/animation/AnimationClock.cpp b/Source/core/animation/AnimationClock.cpp
index 649220690e67a27adf18592ac5d9a46d000c9ff2..d51c95e7aed188da489a296342619fdac98b2841 100644
--- a/Source/core/animation/AnimationClock.cpp
+++ b/Source/core/animation/AnimationClock.cpp
@@ -31,23 +31,52 @@
 #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;
+}
+
 }