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This CL is adding to the RenderText API fuzzer the calls to
  * GetSubstringBounds(...)
  * GetCursorSpan(...)

These calls were missing to trigger the following bug:
  http://crbug.com/1146674

R=gab@chromium.org,msw@chromium.org

Bug: 1146674, 1146674
Change-Id: I2867262bcc8cba97f666bbc084656dd4e1ac7362
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530369Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825960}