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This change adds new functions to AXTextUtils that return the
start and end indices of a string. This function will support a
new API in automation and was implemented in AXTextUtils to
maximize the possibility of shared code.

Bug: 948700
Change-Id: I4c414280910feb1cf6f3180d09e5088264959aba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616383
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667367}