Check MediaDrmBridge before calling OnGenerateKeyRequest().

BUG=297639
TEST=none
R=ddorwin@chromium.org, qinmin@chromium.org

Review URL: https://codereview.chromium.org/24450002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225261 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225261