[pinpoint] Reduce Isolate key hash length from 512 to 256.
[pinpoint] Reduce Isolate key hash length from 512 to 256.

512 is just longer than we need and marginally harder to copy+paste.
For comparison, isolate hashes are SHA-1 hashes (160-bit length)

TBR=sullivan

Review-Url: https://codereview.chromium.org/2952743005

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