This change adds support for logging the following new fields in UKM:
1. initial_url: the initial URL of a source, in cases where the URL of a source
   changes
2. session_id: a counter that's incremented for each browsing session (similar
   to the UMA session_id field)

Logging of both of these fields is gated on field trial params, and is disabled
by default. We'll start logging once the server-side changes for these fields are
in place.

BUG=697944

Review-Url: https://codereview.chromium.org/2722983006
Cr-Commit-Position: refs/heads/master@{#454382}