RFR: 6429812: NPE after calling JTable.updateUI() when using a header renderer + XP L&F [v6]

[Tejesh R]

> _Header_ object not initialized/set when paint() method of `WindowTableHeaderUI` class is executed. The paint() event is executed through explicit call of `JTable.updateUI()` in the regression test. In order to set the _header_ to the _called_ JTable, it is set in the `getTableCellRendererComponent()` method, which in turn makes the _header_ object available during paint event without causing NPE.

Tejesh R has updated the pull request incrementally with one additional commit since the last revision:

  Simplified the Test

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8830/files
  - new: https://git.openjdk.java.net/jdk/pull/8830/files/1ac4d7b5..391e49f1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8830&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8830&range=04-05

  Stats: 88 lines in 1 file changed: 25 ins; 57 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8830.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8830/head:pull/8830

PR: https://git.openjdk.java.net/jdk/pull/8830