Add IntersectionObserverEntry.intersectionRatio attribute.

Merge to 51; original CL:

https://codereview.chromium.org/1872973002

BUG=540528
TBR=ojan@chromium.org
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1879323003

Cr-Commit-Position: refs/branch-heads/2704@{#32}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}
9 files changed