Add null pointer check around volume control delegate.  This was causing
Ash to crash whenever a volume button was pressed in Ash.

TBR sky@ since we only have about 30 minutes to make it in before dev is cut, and the change is low risk.

BUG=313374
TBR=sky

Review URL: https://codereview.chromium.org/53393003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231943 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231943