Chrome Version: Trunk
OS Version: 
URLs (if applicable) :
Other browsers tested:
  Add OK or FAIL after other browsers where you have tested this issue:
     Safari: FAIL (bug to be filed)
    Firefox: FAIL (https://bugzilla.mozilla.org/show_bug.cgi?id=1458816)
    IE/Edge: FAIL (bug to be filed)

What steps will reproduce the problem?
1. Load the test-case at https://bugzilla.mozilla.org/attachment.cgi?id=8972799

What is the expected result?
The test passes, and the iframe resize is properly reflected in the MediaQueryList.

What happens instead of that?
The test fails.

Please provide any additional information below. Attach a screenshot if
possible.