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This in preparation for USER_EVENTS, which is a commit only type and
some fields like version and not useful. We want to save as much
bandwidth as possible, and switching to optional saves us ~2 bytes each
before compression.

BUG=714828

Review-Url: https://codereview.chromium.org/2837993002
Cr-Commit-Position: refs/heads/master@{#468104}