RFR: 8159694: HiDPI, Unity, java/awt/dnd/DropTargetEnterExitTest/MissedDragExitTest.java

[Alexander Zvegintsev]

Window's title height was increased in Gnome/Unity since it was written, so the test tries to initiate drag on frame's header instead of its body. 
Making the frame undecorated solves the issue. 
Testing is green on all platforms.

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Commit messages:
 - initial

Changes: https://git.openjdk.java.net/jdk/pull/8221/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8221&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8159694
  Stats: 11 lines in 3 files changed: 6 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8221.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8221/head:pull/8221

PR: https://git.openjdk.java.net/jdk/pull/8221