This adds the correctness fuzzer harness files to the output
directory guarded by a gn arg.

The switch will be turned on on the builder uploading builds to
clusterfuzz.

BUG=chromium:673246
NOTRY=true
TBR=tandrii@chromium.org,jochen@chromium.org

Review-Url: https://codereview.chromium.org/2623843002
Cr-Commit-Position: refs/heads/master@{#42166}