RFR: 8273986: JEditorPane HTML Demo - Accessibility issues

[Abhishek Kumar]

JAccessWalker was not able to show component tree correctly if we switch pages for HTML content. 

Observation:
The issue observed is that the children are not reported correct for root element when switching of pages happened. The reason behind it is that the `getAccessibleChildrenCount` API is called on the old `accessibleContext` object which return the children count as 0. Whenever we switch the page the children count is recalculated based on the root element but the `accessibleContext `object used to retrieve the child remains unchanged and due to that it return the children count 0.

Solution:

Added a condition check to create a new `accessibleContext `object to find out the children count correctly whenever we switch the pages in JEditorPane. 

Checked with the SwingSet2 JEditorPane demo and it reports well the component tree in JAccessWalker.

Steps to verify:

JBS contains the steps to reproduce the scenario.

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Commit messages:
 - JEditorPane HTML Demo fix

Changes: https://git.openjdk.org/jdk/pull/12707/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12707&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8273986
  Stats: 9 lines in 2 files changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12707.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12707/head:pull/12707

PR: https://git.openjdk.org/jdk/pull/12707