RFR: 8276847: JSException: ReferenceError: Can't find variable: IntersectionObserver

[Jose Pereda]

This PR enables intersection observer support for WebKit, and includes a system test.

-------------

Commit messages:
 - Include html file
 - Enable intersection observer support, including system test

Changes: https://git.openjdk.java.net/jfx/pull/664/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=664&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276847
  Stats: 183 lines in 3 files changed: 180 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/664.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/664/head:pull/664

PR: https://git.openjdk.java.net/jfx/pull/664