[jfx11u] RFR: 8275138: WebView: UserAgent string is empty for first request

[Kevin Rushforth]

Clean backport.

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Commit messages:
 - 8275138: WebView: UserAgent string is empty for first request

Changes: https://git.openjdk.java.net/jfx11u/pull/57/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=57&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275138
  Stats: 51 lines in 4 files changed: 33 ins; 0 del; 18 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/57.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/57/head:pull/57

PR: https://git.openjdk.java.net/jfx11u/pull/57