Bookkeeping for Task IDs for in-tab navigations.

Task IDs are metadata for tracking navigations and relationships between
navigations. They are stored on a NavigationEntry and the bookkeeping
happens via reading signals via NavigationEntryCommitted.

Bug: 958463
Change-Id: If97ae09ac7743ca37ce81bbe401a95e6eeaff5a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592516
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: David Maunder <davidjm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657516}