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The `CreateNextWord*Position` functions were not updating their iterator
in certain edge cases, which caused an infinite loop.  (Notably, the
`CreatePreviousWord*Position` functions *are* updating their iterators
in the same scenarios.)

The new test included in this change would hang before the fix.

The following is a detailed description of the error scenario:
-- Start with a AXTextPosition (at index 0) at a high level in the tree.
-- Call one of the `CreateNextWord*Position` functions.
-- These functions start by getting a "leaf" text position, which drills
   down deeper in the tree.  Specifically, they drill down into a node
   that includes placeholder text (which appears as part of their
   hypertext, but not in the hypertext of the outer node).
-- The function then tries to advance to the next word, and finds a
   valid word stop.
-- The end of this loop compares the new position against the original.
-- operator==/CompareTo must adjust the positions to be relative to the
   same node, which it does using repeated calls to GetParentPosition().
-- GetParentPosition() determines that the text-offset in the deep node
   is not valid in the outer node, so it resets the index to zero.
   (This workaround was added in CL:1661548.)
-- As a result, CompareTo determines that the two nodes are equal.
-- Therefore, we loop in CreateNextWord*Position.
-- Because the iterator was not updated, we loop infinitely.

Bug: 994871
Change-Id: I6e5880bfa7073e2ca672d281fca223e90e5fe288
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758590Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Ian Prest <iapres@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#688611}