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This CL is removing the use of GraphemeIterator.
The function IsValidCursorIndex(...) is moved from
the RenderTextHarfBuzz to RenderText to be with the
other grapheme-related functions.

The main goal is to get rid of the GraphemeIterator.
There are still some uses to be removed.

Bug: 1025561

Change-Id: Ie8483dbc550aee95eb1cd64ca5d3f3e85855ea09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1951445
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722607}