Replace NOTIFICATION_NAV_LIST_PRUNED and NOTIFICATION_NAV_ENTRY_CHANGED
by WebContentsObserver methods. Move SessionService's observation of
those notifications to SessionTabHelper.

Replace NOTIFICATION_TAB_CONTENTS_APPLICATION_EXTENSION_CHANGED with a
direct call from extensions TabHelper to SessionTabHelper.

Remove NOTIFICATION_SESSION_SERVICE_SAVED, the plumbing from
BaseSessionService to enable it and its only use in a test testing that
it is sent.

Bug: 170921, 268984
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: I86aea7b1e63c71d99930a3f604bd5c9686395104
Reviewed-on: https://chromium-review.googlesource.com/630420


Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497281}