Navigation Predictor: Add UMA for action taken

This CL adds logic for determining which URL to prefetch and records
UMA around that. Note that navigation predictor is still running in
dry-run mode, as in no prefetch is actually done.

Change-Id: I620a3c10146f47b3bd2d8b042b54af0a5879fc90
Bug: 903945
TBR: ryansturm@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/1333247
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608209}
7 files changed