RFR: 8276553: ListView scrollTo() is broken after fix for JDK-8089589

[Johan Vos]

After (re)setting the number of elements, make sure to do at least some estimation of the total size.
Added a testcase for this scenario.

-------------

Commit messages:
 - After (re)setting the number of elements, make sure to do at least some estimation of the total size.

Changes: https://git.openjdk.java.net/jfx/pull/683/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=683&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276553
  Stats: 13 lines in 3 files changed: 11 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/683.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/683/head:pull/683

PR: https://git.openjdk.java.net/jfx/pull/683