[webcrypto] Update a comment to indicate that HmacImportParams.hash is required.

This changed upstream in:
https://github.com/w3c/webcrypto/commit/c07a840270334e961e54b4b2ad98f37cd07888cb

Review-Url: https://codereview.chromium.org/2167943002
Cr-Commit-Position: refs/heads/master@{#406803}
1 file changed