Bug 1036287 part 3 - Make GetLocalTimeAt return a nullable time duration; r=dholbert

Once we support arbitrary timelines which can return null current time values,
the local time of an animation can also become null so this patch updates
ElementAnimation::GetLocalTimeAt to return a Nullable<TimeDuration>.
Doing this also allows us to pass the result of GetLocalTimeAt directly to
GetComputedTimingAt.

gfx/layers/composite/AsyncCompositionManager.cpp

@@ -448,7 +448,8 @@ SampleAnimations(Layer* aLayer, TimeStamp aPoint)
     timing.mFillMode = NS_STYLE_ANIMATION_FILL_MODE_BOTH;
 
     ComputedTiming computedTiming =
-      ElementAnimation::GetComputedTimingAt(elapsedDuration, timing);
+      ElementAnimation::GetComputedTimingAt(
+        Nullable<TimeDuration>(elapsedDuration), timing);
 
     NS_ABORT_IF_FALSE(0.0 <= computedTiming.mTimeFraction &&
                       computedTiming.mTimeFraction <= 1.0,