Chromecast: update timestamp requirements
Chromecast: update timestamp requirements

Update the comment requiring CLOCK_MONOTONIC to require
CLOCK_MONOTONIC_RAW which reflects internal timestamp changes.

BUG=internal b/24790163

Review URL: https://codereview.chromium.org/1420923004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355813}

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