allow external tool to resize iframe

refs CNVS-15735

test plan:
 * load a page with an lti tool embedded
 * using the browser's console, post a message to the window, e.g.:
   window.postMessage('{"subject":"lti.frameResize", "height":250}', '*')
 - the lti window should resize to the given height

test plan (alternative):
 * load a page with an lti tool embedded that knows how to issue resize
   messages to canvas
 - the lti window should be resized as the lti tool wishes

Change-Id: Ief192a0552ed96ca829ea1048bdb1a202b377676
Reviewed-on: https://gerrit.instructure.com/6578
Reviewed-by: Nathan Mills <nathanm@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Brad Humphrey <brad@instructure.com>