* Rework mixed content tests.

1.  The existing `iframe` and `form` tests both rely on `<iframe>` elements
    generating `error` events. That doesn't actually happen. This patch
    drops those tests; I'll rewrite them in a subsequent patch.

2.  The test runner didn't properly wrap function calls with `step_func()`,
    which caused some errors to go unseen, as they weren't happening in the
    test's context.

3.  Dropping audio and video samples to lowest common denominator types (wav
    and ogv) in the hopes of executing in a wider range of test harnesses.

4.  Renamed the tests to lead with the type rather than the expected result,
    which makes it easier to find systemic issues like the above.

Change-Id: Iab396d87150578f444039c425dbaa794298c38a7
Reviewed-on: https://chromium-review.googlesource.com/732116
Reviewed-by: Andy Paicu <[email protected]>
Commit-Queue: Mike West <[email protected]>
Cr-Commit-Position: refs/heads/master@{#510774}

* fixup drop 'smallest.wav', as Firefox can't parse it.