RFR: 8276847: JSException: ReferenceError: Can't find variable: IntersectionObserver [v2]

[Jose Pereda]

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

Jose Pereda has updated the pull request incrementally with one additional commit since the last revision:

  Address feedback

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/664/files
  - new: https://git.openjdk.java.net/jfx/pull/664/files/bf0f6437..11d44443

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=664&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=664&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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