Modify and Add tests to OffscreenCanvas promise-based commit()

[xlai-o]

@junov: Please review. Thanks!

This pull request mainly modifies the OffscreenCanvas.commit() tests to handle the latest change in specification about a promise-based commit() in OffscreenCanvas. The changes include:

1. Modify the two existing tests in offscreencanvas.commit.html and offscreencanvas.commit.w.html to promise_tests. In particular, we test the assert pixels after the promise returned by commit() is resolved.
2. Add one more test that assert the pixels in OffscreenCanvas to be non-zero after commit(). This is an adaptation of an existing layout test in Chromium.

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[junov]

Theses tests are great, but let's wait for the promise-returning-commit to be added to the whatwg spec before landing this. There's a conf call tomorrow with participants of the WebGL working group where we'll discuss this. Once multi-vendor support is established, we'll move forward with the spec change, then we can land these tests.