bookmarks: Refactor the code to remove a node from the url set.

Currently we are doing the same thing in two places, so instead of
duplicating the same code in two different places, refactor the code
into a new method that can be called by both places.

BUG=None
TEST=no regressions should be observed when playing with bookmarks.
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/113293012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242339 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242339