Get rid of BookmarkEditor::Show()'s BookmarkEditor::Handler argument.

Still to do: get rid of things which hang on to a handler just so they can pass
it to Show().

BUG=38908
TEST=everything still works

Review URL: http://codereview.chromium.org/1795007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@45813 0039d316-1c4b-4281-b951-d872f2087c98