Implement Navigation Timing Level 2.
Design doc link:
https://docs.google.com/document/d/1DskJn3cKo8RlHxAKJqOSwYlN5mGLOHf3uWo5lwrUWu0/edit#

BUG=504237

Review-Url: https://codereview.chromium.org/2472583003
Cr-Commit-Position: refs/heads/master@{#433765}
15 files changed