RFR: 8171898: [WebView] ScrollBarWidget thickness calculation is not correct

[Arun Joseph]

After [JDK-8157900](https://bugs.openjdk.java.net/browse/JDK-8157900), initializeThickness() moved from
ScrollBarThemeImpl to ScrollBarWidget but the implementation was kept the same by using testSBRef to call
setThickness() for setting ScrollBarTheme.thickness.

Modifying initializeThickness() to set ScrollBarTheme.thickness once and removing testSBRef.

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Commit messages:
 - 8171898: [WebView] ScrollBarWidget thickness calculation is not correct

Changes: https://git.openjdk.java.net/jfx/pull/197/files
 Webrev: https://webrevs.openjdk.java.net/jfx/197/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8171898
  Stats: 22 lines in 2 files changed: 0 ins; 15 del; 7 mod
  Patch: https://git.openjdk.java.net/jfx/pull/197.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/197/head:pull/197

PR: https://git.openjdk.java.net/jfx/pull/197