Clear |drm_bridges_| in BrowserMediaPlayerManager::DestroyAllMediaPlayers().

BUG=299853
TEST=No browser crash after previous renderer crash.
R=qinmin@chromium.org

Review URL: https://codereview.chromium.org/25146002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225989 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225989