Remove unused property.

This was added long ago in https://codereview.chromium.org/357005/ , but
wasn't used in patch set 1 in that review either.

No behavior change.  Fixes a new clang warning.

BUG=323668

Review URL: https://codereview.chromium.org/120673002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242577 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242577