Sprinkle some CHECKs in WebMediaPlayerImpl to track down some confusing crashes.

It seems |pipeline_| has been deleted but |this| hasn't, which is a really confusing combo.

BUG=341184,341186

Review URL: https://codereview.chromium.org/131033005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249671 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249671