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When diving up the input into 4 chunks, it was skipping over the final
character of each chunk. This happened even if a chunk included all
remaining characters, resulting in a StringPiece that started
beyond the end of the string. This resulted in a 0-length StringPiece,
and things worked fine for years, until a CHECK was added catching this
this brokenness.

This CL switches the fuzzer over to using FuzzedDataProvider, which
doesn't have this issue.

Fixed: 1123036
Change-Id: If17e26d065a157f8d401a7e4afc06c769c8670f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382063Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802788}