Web Animations: Set the DocumentTimeline's zero time when the Document reaches the 'loading' phase

Source/core/animation/css/CSSAnimations.cpp

Index: Source/core/animation/css/CSSAnimations.cpp
diff --git a/Source/core/animation/css/CSSAnimations.cpp b/Source/core/animation/css/CSSAnimations.cpp
index f3d06f28beb6181985832c542485800f6c1c493a..b3358ea2f2663d0ff71422e4fbc9c34b39e94fe6 100644
--- a/Source/core/animation/css/CSSAnimations.cpp
+++ b/Source/core/animation/css/CSSAnimations.cpp
@@ -189,6 +189,7 @@ void CSSAnimations::update(Element* element, const RenderStyle* style)
                 Timing timing;
                 timingFromAnimationData(animationData, timing);
                 OwnPtr<CSSAnimations::EventDelegate> eventDelegate = adoptPtr(new EventDelegate(element, animationName));
+                ASSERT(!isNull(element->document()->timeline()->currentTime()));
                 // FIXME: crbug.com/268791 - Keyframes are already normalized, perhaps there should be a flag on KeyframeAnimationEffect to skip normalization.
                 m_animations.set(animationName.impl(), element->document()->timeline()->play(
                     Animation::create(element, KeyframeAnimationEffect::create(keyframes), timing, eventDelegate.release()).get()).get());