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This is done for three reasons:
1. The two methods don't really check if the next / previous anchor is a text node.
They simply move to the next / previous leaf text position
assuming that every leaf node is a text node.
2. There is already a set of corresponding CreateNext/PreviousLeafTreePosition
methods.
3. There is a similar method `CreateNextLeafTextPosition(bool*)`, used for paragraph boundaries. Making
that method to be an overload of CreateNextLeafTextPosition is clearer, since they both move in
the same way through the AX tree.

Split out from a larger patch that makes AXPosition work with a forest
of accessibility trees at:
https://chromium-review.googlesource.com/c/chromium/src/+/2595454

AX-Relnotes: n/a.

TBR=dmazzoni@chromium.org, aleventhal@chromium.org

Bug: 1049261
Change-Id: Idc56c19f908fde8b145a55fe542ee637ae4cbd44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2598625Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838567}