SyncedTabDelegate now just returns the data used by Sessions directly, and
moves all content interaction into its implementation.

As part of this, synced_tab_delegate_desktop becomes obsolete, as the
logic to decide which type of synced tab delegate is now just inlined in the
NotificationServiceSessionsRouter. In addition, SyncedSessionsUtil has
been distributed throughout where it was actually used, and ShouldSyncURL
is now a member off the new SyncSessionsClient interface.

BUG=512468, 512471

Review URL: https://codereview.chromium.org/1408643002

Cr-Commit-Position: refs/heads/master@{#356240}