RFR: 8311805: Clean up ScrollPane: drop redundant initialiser, mark scroller final

[Alexey Ivanov]

A small clean-up of `awt.ScrollPane` code:

1. Remove the redundant initialiser for `vs` in `layout` and move the declaration to where it's used. (One less `Dimension` object is created.)
2. Mark the `scroller` field of the `PeerFixer` class as `final`.

Perhaps, the  `PeerFixer` class could be `final` too.

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Commit messages:
 - 8311805: Clean up ScrollPane: drop redundant initialiser, mark scroller final

Changes: https://git.openjdk.org/jdk/pull/14827/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14827&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311805
  Stats: 4 lines in 1 file changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/14827.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14827/head:pull/14827

PR: https://git.openjdk.org/jdk/pull/14827