RFR: 8190411: NPE in SliderSkin:140 if Slider.Tooltip.autohide is true [v2]

[Karthik P K]

> Cause: When slider is dragged for first time after tooltip appears, setOnMousePressed event was not invoked, hence dragStart was null in the subsequently invoked event handler (setOnMouseDragged).
> 
> Fix: Initialized dragStart in initialize method.
> 
> Test: Added system test to validate the fix.
> 
> Note: Initializing dragStart in layoutChildren method as suggested in the bug was causing side effects. Hence initialized dragStart in initialize method.

Karthik P K has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/965/files
  - new: https://git.openjdk.org/jfx/pull/965/files/289fda52..86e7823c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=965&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=965&range=00-01

  Stats: 62 lines in 1 file changed: 23 ins; 25 del; 14 mod
  Patch: https://git.openjdk.org/jfx/pull/965.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/965/head:pull/965

PR: https://git.openjdk.org/jfx/pull/965