Bug 1436659 - Add Animation::GetCurrentTimeForHoldTime helper; r=hiro

We will use this to calculate the unconstrained current time when we implement
pending playback rate changes.

MozReview-Commit-ID: 7GlmHiGeXF6

dom/animation/Animation.cpp

@@ -298,11 +298,12 @@ Animation::SetStartTime(const Nullable<TimeDuration>& aNewStartTime)
 
 // https://drafts.csswg.org/web-animations/#current-time
 Nullable<TimeDuration>
-Animation::GetCurrentTime() const
+Animation::GetCurrentTimeForHoldTime(
+  const Nullable<TimeDuration>& aHoldTime) const
 {
   Nullable<TimeDuration> result;
-  if (!mHoldTime.IsNull()) {
-    result = mHoldTime;
+  if (!aHoldTime.IsNull()) {
+    result = aHoldTime;
     return result;
   }