Expand zero-area target elements when computing intersections.

Based on spec discussion:

https://github.com/WICG/IntersectionObserver/issues/69

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

Review URL: https://codereview.chromium.org/1649283002

Cr-Commit-Position: refs/heads/master@{#373035}
3 files changed