Bug 1109390 part 3 - Clear the start time when playing a player; r=jwatt

This brings us into line with the algorithm in:

  https://w3c.github.io/web-animations/#play-an-animation

which makes the other patches in this series easier to compare with
the specification.

dom/animation/AnimationPlayer.cpp

@@ -442,6 +442,9 @@ AnimationPlayer::DoPlay()
   // Clear ready promise. We'll create a new one lazily.
   mReady = nullptr;
 
+  // Clear the start time until we resolve a new one
+  mStartTime.SetNull();
+
   mIsPending = true;
 
   nsIDocument* doc = GetRenderedDocument();