Relanding WebRTC peerconnection iframe test.

      This is a version of perkj's test in codereview.chromium.org/1073783003
which also covers a full peer connection call. It does not attempt to
verify the case where we nuke the iframe though since we don't know
what will happen past the gUM call in that call. The WebRTC call in
the iframe will just get torn down so there's not much we can test.

This fixes a bug in the test from the original patch, where we would
negotiate the call twice because of an iframe.onload quirk. This
actually worked except on Windows, but it's a degenerate use case we
should not test (except maybe in a fuzzer).

BUG=479093

Review URL: https://codereview.chromium.org/1161403002

Cr-Commit-Position: refs/heads/master@{#332586}