RFR: 8272808: Update constant collections to use the new immutable collections - leftovers

[Kevin Rushforth]

On Tue, 21 Sep 2021 11:13:06 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

> Replacements of more immutable collections.
> 
> One thing I found was that the field `idMap` in `com.sun.java.scene.web.WebViewHelper.WebView` seems unused. I can remove it if that's indeed the case.

This is a pretty simple change, but a second pair of eyes would be helpful.

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PR: https://git.openjdk.java.net/jfx/pull/627