RFR: 8362873: Regression in BorderPane after JDK-8350149

[John Hendrikx]

This PR fixes a regression introduced in #1723

It reverts the changes made in the BorderPaneTest with new comments explaining why the values are what they are.  It only fixes the regression that was introduced, but it can be said that the calculation is probably also incorrect when a vertical biased control is encountered (the `getAreaWidth` and `getAreaHeight` in `BorderPane` were, and are still not using the same way to calculate their respective sizes).  See the ticket for more details and screenshots.

Let me know if I should only address the regression introduced, or also include the fix for the bias calculation for vertical bias controls.

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Commit messages:
 - Fix regression

Changes: https://git.openjdk.org/jfx/pull/1850/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1850&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8362873
  Stats: 50 lines in 2 files changed: 37 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jfx/pull/1850.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1850/head:pull/1850

PR: https://git.openjdk.org/jfx/pull/1850