[jfx11u] RFR: 8277133: Dragboard contents retrieved all over again during a DND process on WebView

[Johan Vos]

Clean backport of 
8277133: Dragboard contents retrieved all over again during a DND process on WebView

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Commit messages:
 - 8277133: Dragboard contents retrieved all over again during a DND process on WebView

Changes: https://git.openjdk.java.net/jfx11u/pull/64/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=64&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277133
  Stats: 101 lines in 5 files changed: 91 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/64.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/64/head:pull/64

PR: https://git.openjdk.java.net/jfx11u/pull/64