[ios] ARCMigrate i/c/b/tabs/tab_model_synced_window_delegate*.

Convert TabModelSyncedWindowDelegate to use WebStateList instead
of TabModel and convert the file to ARC (i.e. add the ARC header
guard).

BUG=None

Review-Url: https://codereview.chromium.org/2845353002
Cr-Commit-Position: refs/heads/master@{#470974}
5 files changed