[jfx11u] RFR: 8211308: Support HTTP/2 in WebView

[Kevin Rushforth]

Clean backport of HTTP/2. This is enabled by default only with running with JDK 12 or later.

I have tested this with both JDK 11, where I verified that it HTTP/2 is not enabled, and JDK 15, where I verified that it HTTP/2 is enabled.

As an FYI, I tested this patch in my [test-kcr-11.0.13](https://github.com/kevinrushforth/jfx11u/tree/test-kcr-11.0.13) branch, which has the aggregate set of patches for the following bugs: [JDK-8211308](https://bugs.openjdk.java.net/browse/JDK-8211308), [JDK-8268915](https://bugs.openjdk.java.net/browse/JDK-8268915), [JDK-8269147](https://bugs.openjdk.java.net/browse/JDK-8269147), [JDK-8269131](https://bugs.openjdk.java.net/browse/JDK-8269131), [JDK-8268849](https://bugs.openjdk.java.net/browse/JDK-8268849), [JDK-8270479](https://bugs.openjdk.java.net/browse/JDK-8270479), [JDK-8272329](https://bugs.openjdk.java.net/browse/JDK-8272329).

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Commit messages:
 - 8211308: Support HTTP/2 in WebView

Changes: https://git.openjdk.java.net/jfx11u/pull/30/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=30&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8211308
  Stats: 1166 lines in 14 files changed: 881 ins; 217 del; 68 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/30.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/30/head:pull/30

PR: https://git.openjdk.java.net/jfx11u/pull/30