Add root margin support for IntersectionObserver.

Depends on:

https://codereview.chromium.org/1553773002/

BUG=540528
R=ojan@chromium.org,esprehn@chromium.org

Review URL: https://codereview.chromium.org/1559593002

Cr-Commit-Position: refs/heads/master@{#369015}
6 files changed