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Prior to this patch, TabContentsSyncedTabDelegate was itself a tab
helper (WebContentsUserData) that was created on all platforms, with the
whole purpose to reuse a partial implementation of
sync_sessions::SyncedTabDelegate.

This was a bit weird because
a) The life cycle diverges across platforms: created lazily on Android,
   early otherwise.
b) On Android there was no real need to register a tab helper.
c) On Android, it's weird to reason about two tab delegates coexisting,
   one of which is a proxy but also overrides some logic.

Instead, let's make TabContentsSyncedTabDelegate a base class that is
*NOT* a WebContentsUserData, and let platform-specific subclasses
decide whether they want to register as WebContentsUserData, as well
as which logic they need to override.

This also removes code that is dead on some platforms, because the
base class is abtract and doesn't implements all methods (as opposed
to a proxy object which cannot be abstract).

Bug: 851905
Change-Id: I305ad85e2392fdd2ce428c0464b7750ac599babf
Reviewed-on: https://chromium-review.googlesource.com/1097405
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567213}