Fix UnindexedRulesetWriter finishing.

This CL fixes the bug where UnindexedRulesetWriter doesn't trim the
CodedOutputStream on Finish, resulting in the output having some extra
uninitialized bytes at the end.

BUG=609747

Review-Url: https://codereview.chromium.org/2279803002
Cr-Commit-Position: refs/heads/master@{#414762}
4 files changed