frame_size_tests: reduce default 'large' frame size

VP9FrameSizeTestsLarge exposed an integer overflow in the VP9 encoder,
for now reduce the size to allow the tests to clear and prevent further
regressions.
4096x4096 -> 4096x2160

this should be restored after the bug is fixed:
https://code.google.com/p/webm/issues/detail?id=828

Change-Id: I47fdf0648f1d9a3951f731bbf0b727f85ada4fa1