Tests sporadically failing with "Element matching css "#edit-submit" not found" on Media tests

Since switching to the 'trusty' beta on Travis CI (per #2869464: All Chrome tests failing with "Could not open connection: An unknown server-side error"), we're getting sporadic failures on the Media tests with:

And I click "Browse"
And I switch to the frame "mediaBrowser"
And I attach the file "test-sm.png" to "files[upload]"
And I press "Next"
Then I should see "Destination"
When I select the radio button "Public local files served by the webserver."
And I press "Next"                                
Then I should see a "#edit-submit" element
      Element matching css "#edit-submit" not found. (Behat\Mink\Exception\ElementNotFoundException)
    │
    │  Screenshot uploaded to http://i.imgur.com/JAnG5v8.png
    │  
    │
    └─ @AfterStep # TestSubContext::afterStepTakeScreenshot()

See https://travis-ci.org/panopoly/panopoly/jobs/231687183#L1133

It works most of the time, but fails a lot, not always on the same test, but always on that same step.