fix a bug where all retrieve launches were breaking out of their frame

Test Plan:
 - launch a tool using /courses/:course_id/external_tools/retrieve?url=:launch_url
   * it should stay in the canvas frame
 - launch a tool using /courses/:course_id/external_tools/retrieve?url=:launch_url&borderless=1
   * it should break out of the canvas frame

Change-Id: Icd497c95f2794e53fc288b9f35fd024451c33931
Reviewed-on: https://gerrit.instructure.com/30863
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Eric Berry <ericb@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>