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The call to wcslen on a StringPiece is incorrect. There is no guarantee that
the string is NUL terminated.

As stated on crbug/624905:
 "Problem is that gfx::RenderTextHarfBuzz::ShapeRun is asking for fallback for
  an empty string but claims it's of length 1. This confuses DirectWrite and it
  crashes. Can easily work around the crash by checking for empty string, but
  still need to investigate why gfx::RenderTextHarfBuzz::ShapeRun is getting
  confused."

The reason of the code was to avoid a broken case with ShapeRuns.
The empty string is no longer accepted in any GetFallbackFont(...)

Bug: 624905

Change-Id: I32e2583c0b87084ece549bf18b611ed7ae8f8c72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1612045
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660086}