The "max_len" argument for fuzzer targets is deprecated. We need to
enforce the limit internally.

R=ahaas@chromium.org

Bug: chromium:894939
Change-Id: I2206bc63d5e39f1aa189e11042a6a0bbcca31b0d
Reviewed-on: https://chromium-review.googlesource.com/c/1299020Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56998}