[Extensions] Check if the render frame created the document for app bindings

      In the switch to render frames, there's now a chance that the document element
has been created before the JS sets up the callback. Check if it's done before
trying to wait on it.

BUG=511042

Review URL: https://codereview.chromium.org/1254453005

Cr-Commit-Position: refs/heads/master@{#340037}