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This reverts commit 42c62276.

Reason for revert: suspect for browser crashes in canary as per
crbug.com/853049.

Original change's description:
> Avoid a proxy tab helper on Android for sync delegation
> 
> 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}

TBR=avi@chromium.org,treib@chromium.org,mastiz@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 851905,853049
Change-Id: I71a78fa8fd50b551c5ae50cb64f1fe5e033383e5
Reviewed-on: https://chromium-review.googlesource.com/1102537Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567662}