This is the first step to introduce task in Chrome, which links and
groups navigations. Each navigation has a task id and a list of its
ancestor's task ids. A navigation's task id is TabNavigation's global
id when the navigation is first visited and then reused by going
back/forward visits. The task ids are created and sent to backend in
session sync.

And for now we only track navigation relationship in a tab.

BUG=707978

Review-Url: https://codereview.chromium.org/2776633003
Cr-Commit-Position: refs/heads/master@{#466188}