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

[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.

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

Commit messages:
 - header object set
 - Merge remote-tracking branch 'upstream/master' into branch_6429812
 - Merge remote-tracking branch 'upstream/master'
 - Merge branch 'master' of github.com:TejeshR13/jdk
 - Merge branch 'openjdk:master' into master
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge branch 'master' of github.com:TejeshR13/jdk
 - Initial Commit.

Changes: https://git.openjdk.java.net/jdk/pull/8830/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8830&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-6429812
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 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