Add a unit test for DisplayList recording vs. the opaque region tracker.

This bug was fixed a few weeks ago in
https://src.chromium.org/viewvc/blink?revision=182002&view=revision,
but it had no unit test.

BUG=418659

Review URL: https://codereview.chromium.org/616403002

git-svn-id: svn://svn.chromium.org/blink/trunk@183114 bbb929c8-8fbe-4397-9dbb-9b2b20218538