Minimum viewport size

[annevk]

As discussed in web-platform-tests/wpt#45161 and prior to that in web-platform-tests/wpt#7644 and elsewhere there's minimum viewport size assumptions. As @jgraham points out we should probably put those in wptrunner.

[gsnedders]

For reftests we expect exactly an 800x600 viewport, and I think we should probably just make that assumption for all tests.

[jgraham]

Requiring an exact size means that non-reftests have the same problem on mobile that reftests have i.e. you either can't run them or have to run in some very nonstandard viewport.

I tend to think that for testharness tests we want to discourage writing tests that depend on the exact dimensions, but it probably does make sense to ensure some minimum dimensions so that tests have something reasonable to rely on.