Web Animations: Compositor timeOffset should not be scaled

This also seems to indicate that it's not yet possible to composite an
animation with a negative playback rate and infinite duration.

BUG=422432

Review URL: https://codereview.chromium.org/667633003

git-svn-id: svn://svn.chromium.org/blink/trunk@183957 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/ManualTests/animation/compositor-change-playback-rate.html

+<style>
+div {
+  height: 100px;
+  width: 100px;
+  background: blue;
+}
+</style>
+<p>
+The four squares should make identical rotations at different rates without jumping.
+<div id="target1"></div>
+<div id="target2"></div>
+<div id="target3"></div>
+<div id="target4"></div>
+<script>
+var player1 = target1.animate([
+  {transform: 'none'},
+  {transform: 'rotate(90deg)'}
+], {duration: 1000, iterations: 200000});
+
+var player2 = target2.animate([
+  {transform: 'none', background: 'blue'},
+  {transform: 'rotate(90deg)', background: 'blue'}
+], {duration: 1000, iterations: 200000});
+
+var player3 = target3.animate([
+  {transform: 'none'},
+  {transform: 'rotate(90deg)'}
+], {duration: 1000, iterations: Infinity});
+
+var player4 = target4.animate([
+  {transform: 'none', background: 'blue'},
+  {transform: 'rotate(90deg)', background: 'blue'}
+], {duration: 1000, iterations: Infinity});
+
+setInterval(function() {
+  var playbackRate = (Math.random() - 0.5) * 5;
+  player1.playbackRate = playbackRate;
+  player2.playbackRate = playbackRate;
+  player3.playbackRate = playbackRate;
+  player4.playbackRate = playbackRate;
+}, 100);
+</script>

third_party/WebKit/Source/core/animation/AnimationPlayer.cpp

@@ -37,6 +37,7 @@
 #include "core/events/AnimationPlayerEvent.h"
 #include "core/frame/UseCounter.h"
 #include "platform/TraceEvent.h"
+#include "wtf/MathExtras.h"
 
 namespace blink {
 
@@ -597,7 +598,7 @@ void AnimationPlayer::setOutdated()
 
 bool AnimationPlayer::canStartAnimationOnCompositor()
 {
-    if (m_playbackRate == 0)
+    if (m_playbackRate == 0 || (std::isinf(sourceEnd()) && m_playbackRate < 0))
         return false;
 
     return m_timeline && m_content && m_content->isAnimation() && playing();
@@ -612,6 +613,7 @@ bool AnimationPlayer::maybeStartAnimationOnCompositor()
     double timeOffset = 0;
     if (std::isnan(startTime)) {
         timeOffset = m_playbackRate < 0 ? sourceEnd() - currentTimeInternal() : currentTimeInternal();
+        timeOffset = timeOffset / fabs(m_playbackRate);
     }
     return toAnimation(m_content.get())->maybeStartAnimationOnCompositor(startTime, timeOffset, m_playbackRate);
 }