[web-animations-2] Auto-aligning the start time should apply a pending playback rate (#14175)

Throughout the Web Animations spec we typically apply the pending playback rate of an animation when setting its start time. It is an oversight not to do so within the procedure to auto-aligning the start time.

Addresses #14173.

Author: graouts

web-animations-2/Overview.bs

@@ -818,6 +818,8 @@ The procedure for <dfn export for="animation">calculating an auto-aligned start
     1.  Set [=animation/start time=] to |start offset| if [=effective playback rate=] &ge; 0,
         and |end offset| otherwise.
 
+    1.  [=Apply any pending playback rate=] on |animation|.
+
     1.  Clear [=hold time=].