Fix iframe issue for XHProf DarkConsole plugin

Summary:
While sort of gross, this seems fairly reasonable overall? I guess?

(This patch clearly does more good than harm, although it could just do the good
without the harm.)

Test Plan: Clicked XHProf links from the frame and from the /xhprof/ tool.

Reviewers: btrahan, aran, jungejason, ide

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T453

Differential Revision: https://secure.phabricator.com/D1498