Add text justifying why we call MediaControls::Reset()

After investigating we do need to call MediaControls::Reset(). Many of
the state computations inside of MediaControlsImpl key off of the
controls visibility, so a full state reset is needed in order to put
the controls in the right state.

It's possible we could create a more scoped version of Reset() just for
the controls that shift with the visible state, but there don't seem to
be any issues with the bigger hammer that is Reset().

Bug: 448699375
Change-Id: Ic17991a13151e023a269497b90ab8fb9e5aeef31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7007030
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1524830}