Comment cleanup on BookmarkTagModel.

BUG=crbug/11526587

Review URL: https://codereview.chromium.org/59483003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243276 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243276