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No behavioral differences: a new struct (TrackedSession) is introduced
to represent what the tracker knows about a session, which is an
extended version of SyncedSession.

The set |tab_node_ids| is moved away from SyncedSession to the new
struct, because there's nothing that depends on it except the tracker
itself.

Somewhat unrelated, minor changes are introduced in the API as have
been suggested in code reviews.

Bug: 681921
Change-Id: Ife4c215b65dfbeb97d5c70fd1f984986b9938a8f
Reviewed-on: https://chromium-review.googlesource.com/983773
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546755}