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This change is in preparation for a fix to the right-click selection behavior
on Mac.

This change also has RenderTextHarfBuzz EnsureLayout() when getting a substring
range.

Bug: 856609
Change-Id: I85c81c630fc2e6ac79d01a7a25a6f3e30756fdb5
Reviewed-on: https://chromium-review.googlesource.com/1156844
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579820}