Add a comment to dissuade addition of browser options.

BUG=NONE
TEST=comment only.
NOTRY=True

Review URL: https://codereview.chromium.org/27045006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254537 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254537