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

[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."

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Commit messages:
 - Window.close(), Fix for localstoarge

Changes: https://git.openjdk.java.net/jfx/pull/703/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=703&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255940
  Stats: 128 lines in 3 files changed: 118 ins; 9 del; 1 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