IntersectionObserver: notify when root or target is removed.

If root and target were previously intersecting, and then DOM is
modified to remove root or target, or to make target not a
containing block descendant of root, send a notification.

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

Review URL: https://codereview.chromium.org/1889053002

Cr-Commit-Position: refs/heads/master@{#387719}
5 files changed