This CL makes a fuzzer out of the cctest
test-multiple-return/ReturnMultipleRandom. The fuzzer creates a
CallDescriptor with input parameters and returns, and a function which
maps input parameters to returns. The fuzzer then calls this function
with a wrapper which checks that the correct mapping happened.

R=clemensh@chromium.org

Change-Id: Ib89c4063638baae69540a44486d7b2e9d13f8c1f
Reviewed-on: https://chromium-review.googlesource.com/859768Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50532}