As a first step towards making CSS animations and CSS transitions use the same
data structures, this patch aligns their behavior with regards to start time and
delay handling.
Previously, ElementAnimation objects maintained separate mStartTime and mDelay
members whilst ElementPropertyTransition objects maintained a single mStartTime
property that incorporated the delay. This patch adds an mDelay member to
ElementPropertyTransition and stores the delay and start time separately.
Calculations involving ElementPropertyTransition::mStartTime are adjusted to
incorporate mDelay.
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