Scroll interruption tests for CSSOM View smooth scroll

This adds a SmoothScrollInterruptionTest class and tests using
this class.

BUG=243871

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178387

Review URL: https://codereview.chromium.org/398043005

git-svn-id: svn://svn.chromium.org/blink/trunk@178481 bbb929c8-8fbe-4397-9dbb-9b2b20218538