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The string-based version of this function takes const inputs, but the
vector-based one did not. Align them so that the vector-based function
is more general.

Change-Id: I25ee663cc1d35fa9b0bb6cffc33dd0cd8b460423
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404067Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806720}