Add layout tests for universal overflow scrolling

This cl adds layout tests that stress the corner cases that the old composited
scrolling path could not handle. Namely, situations where promoting would cause
stacking and clipping errors.

This is blink-side patch 1 of https://docs.google.com/a/chromium.org/document/d/1Ln57_v6_4RmuuvtWnpsfBOUeDFrPsYNFW13dwuQ8UD8.

BUG=254435

Review URL: https://chromiumcodereview.appspot.com/22620002

git-svn-id: svn://svn.chromium.org/blink/trunk@156725 bbb929c8-8fbe-4397-9dbb-9b2b20218538