Scroll interruption tests for CSSOM View smooth scroll

This adds a SmoothScrollInterruptionTest class and tests using
this class.

BUG=243871

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

git-svn-id: svn://svn.chromium.org/blink/trunk@178387 bbb929c8-8fbe-4397-9dbb-9b2b20218538