[M108 Migration][MM] Fix crash when the browser is terminated when playing video.

[M108 Migration][MM] Fix crash when the browser is terminated when playing video.

1. MediaPlayerESPlusPlayer should be released before destroyed.
2. WebContents is sometimes destroyed before TizenRendererImpl is destroyed.
   It causes TizenRendererImpl accesses dangling pointer.
3. buffering_callback_ in BufferObserverImpl should be null-checked.

Reference:
https://review.tizen.org/gerrit/276393/

Change-Id: I772c80ce9a0f96c5cdc6db6c31aff3465a54eb88
Signed-off-by: Suhaspoornachandra <s.poornachan@samsung.com>