RFR: 8255940: localStorage is null after window.close() [v4]

[Jay Bhaskar]

> Issue: The current implementation of DOMWindow ::localStorage(..) returns null pointer in case of page is being closed. 
> Fix: It should not return nullptr , as per the [w3c web storage spec](https://www.w3.org/TR/2016/REC-webstorage-20160419/) 
> "User agents should expire data from the local storage areas only for security reasons or when requested to do so by the user. User agents should always avoid deleting data while a script that could access that data is running."

Jay Bhaskar has updated the pull request incrementally with one additional commit since the last revision:

  Add Review Changes

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/703/files
  - new: https://git.openjdk.java.net/jfx/pull/703/files/a8647839..3f6e815e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=703&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=703&range=02-03

  Stats: 21 lines in 2 files changed: 14 ins; 5 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/703.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/703/head:pull/703

PR: https://git.openjdk.java.net/jfx/pull/703