Domain Reliability: Add sample_rate field to Beacon

Domain Reliability reports details of URL requests with a certain
probability (sample rate). By including the sample rate in the
beacon, the collector can estimate how many other requests (on
average) were not reported.

BUG=567307

Review URL: https://codereview.chromium.org/1497803004

Cr-Commit-Position: refs/heads/master@{#364446}
6 files changed