The correctness fuzzer executes the input array in two different
execution engines and compares the results. If the results don't match,
the correctness fuzzer crashes.

Since the crash signature is always the same if the results don't match,
cluster fuzz would group all inputs which lead to non-matching results.

To avoid the grouping a base64 hash has to be appended to the crash
signature. This CL changes the text which is appended to the crash
signature to a base64 hash.

Note that I do not create a base64 hash directly because the base64
class is not available in V8. Instead I create a string which looks
like a base64 hash.

R=mmoroz@chromium.org, aarya@chromium.org, titzer@chromium.org

Review-Url: https://codereview.chromium.org/2390233002
Cr-Commit-Position: refs/heads/master@{#39953}