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1. In order to collect the anchors between the start and end positions
   of an `AXRange`, an incorrect condition is evaluating if the start of
   the current anchor is equal to its end position, expecting such case
   to only be present when we reach the end of the `AXRange`.

   Such assumption does not hold for anchors of objects invisible to
   the text representation (such as checkboxes, images, etc.), since the
   anchor is empty and its start and end positions are the same.

   Refactoring `AXRange::GetAnchors` to cover such case.

2. Introducing unit tests for `AXRange::GetAnchors`.

3. Adding unit test coverage in `AXPlatformNodeTextRangeProviderTest`
   for the issue in `GetBoundingRectangles` which led to this bug.

Bug: 928948
Change-Id: I73ef429b7fb5043cfb535ce05909ac243b31aef1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1595545
Commit-Queue: Ethan Jimenez <ethavar@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659701}