Add ScrollTree builder and unit test
Remove ScrollBlocksOn from ScrollTree:
It will be moved from the code base soon, will work
to integrate the "passive event handler" if that
depends on layer tree hierarchy.

BUG=568830
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1626513003

Cr-Commit-Position: refs/heads/master@{#371914}
11 files changed