Overflow tests

While working on overflow, I've discovered corner cases that were not covered by existing tests. The conformance to spec among browsers was also not very good. So I decided to create a small test suite focussed on overflow in CSS box model.

I am talking to w-p-t folks about moving this to w-p-t. I'd like to commit it here right now so layoutng overflow can be tested exhaustively.

Currently:
Edge passes all tests
FF fails overflow-basic-004.html
Chrome fails overflow-basic-002.html, overflow-basic-004.html, and overflow-transform-002.html

I've filed bugs for failing test cases.

BUG=635619

Review-Url: https://codereview.chromium.org/2895033002
Cr-Commit-Position: refs/heads/master@{#474433}
11 files changed