Add iframe support for test runner

[jcxia43]

This is to add test runner an optional feature of running tests in an iframe while keeping running tests in a top-level browsing context as default.
It makes the test runner work smoothly in Web Runtime, and benefits reftests and manual tests running on mobile devices by saving tab/window switching operations

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/955

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[jcxia43]

@jgraham , Would you please review it? Thanks indeed.

[jgraham]

I'm pretty sure that this is a bad idea. Some tests depend on being in a top level browsing context, so it's wrong to run them in an iframe. For running on mobile you really want something automated for all the non-manual tests, in the style of [1](only runs testharness tests on mobile at the moment but should eventually support reftests too. Also very beta and entirely Mozilla-specific)

[1] https://pypi.python.org/pypi/wptrunner

[jcxia43]

Thanks, got it.