RFR: 7148092: [macosx] When Alt+down arrow key is pressed, the combobox popup does not appear.

[Prasanta Sadhukhan]

When comboBox have focus, pressing Alt+Down (or Up) should render the popup which is not happening for Aqua L&F.

Fix is to add toggleAction for Alt+Down/Up keys as has been done for [MetalLookAndFeel](https://github.com/openjdk/jdk/blob/master/src/java.desktop/share/classes/javax/swing/plaf/metal/MetalLookAndFeel.java#L912)

closed problemlisted test is used to check the fix.

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

Commit messages:
 - 7148092: [macosx] When Alt+down arrow key is pressed, the combobox popup does not appear.

Changes: https://git.openjdk.org/jdk/pull/10014/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10014&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7148092
  Stats: 14 lines in 2 files changed: 14 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10014.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10014/head:pull/10014

PR: https://git.openjdk.org/jdk/pull/10014