RFR: 8278759 : PointerEvent: buttons property set to 0 when mouse down

[Hima Bindu Meda]

Basically, buttons property is a mask which represents the button/buttons clicked on the mouse.
    It is observed that event.buttons property is set to 0 when there is mouse press or drag event.This behaviour is observed only with javafx webView.Other browsers set the buttons property to 1, when there is mouse press or drag.
     The issue happens because the buttons property is not updated in the framework.
     Added implementation to update and propagate the buttons property from javafx platform to native webkit.Added a robot test case for the same.
   Performed sanity testing with the added implementation and the buttons property is compliant with the specification mentioned in https://w3c.github.io/pointerevents/#the-buttons-property.

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Commit messages:
 - Resolve formatting errors
 - Enable buttons property for mouse events

Changes: https://git.openjdk.java.net/jfx/pull/742/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=742&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278759
  Stats: 307 lines in 8 files changed: 296 ins; 1 del; 10 mod
  Patch: https://git.openjdk.java.net/jfx/pull/742.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/742/head:pull/742

PR: https://git.openjdk.java.net/jfx/pull/742