RFR: 6513512: MetalLookAndFeel.initClassDefaults does not install an entry for MetalMenuBarUI

[Prasanta Sadhukhan]

Spec for [MetalLookAndFeel](https://github.com/openjdk/jdk/blob/master/src/java.desktop/share/classes/javax/swing/plaf/metal/MetalLookAndFeel.java#L247)
says:
"...MetalLookAndFeel registers an entry for each of the classes
in the package javax.swing.plaf.metal that are named MetalXXXUI.
The string XXX is one of Swing's uiClassIDs. For the uiClassIDs
that do not have a class in metal, the corresponding class in
javax.swing.plaf.basic is used. For example, metal does not
have a class named "MetalColorChooserUI", as such,
javax.swing.plaf.basic.BasicColorChooserUI is used".

There is class MetalMenuBarUI, but the method populates given defaults table with the value
"javax.swing.plaf.basic.BasicMenuBarUI".

Added entry for MetalMenuBarUI..
CI tests including JCK tests are ok.

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Commit messages:
 - 6513512: MetalLookAndFeel.initClassDefaults does not install an entry for MetalMenuBarUI

Changes: https://git.openjdk.org/jdk/pull/11646/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11646&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6513512
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11646.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11646/head:pull/11646

PR: https://git.openjdk.org/jdk/pull/11646