Bug 1290420 Part 5: Change test function waitForViewportResizeTo to check viewport size. r=gl

Several tests of RDM use the helper function waitForViewportResizeTo. This patch changes that function to check the viewport size, not the content size, and to wait on either the content-resized event or the newly-defined viewport-resized event, which is fired on resize when viewport override is active.

Differential Revision: https://phabricator.services.mozilla.com/D7202