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This is a reland of 42c62276

The underlying issue in SyncedTabDelegateAndroid::GetSessionId()
has been reverted, which introduced an accidental behavioral
difference in the original version, causing crashes on Android
for placeholder tabs (i.e. tabs without WebContents).

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

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}

Bug: 851905
Change-Id: I9e060ecf1272fec1ceebd4353fd390b95e64b669
Reviewed-on: https://chromium-review.googlesource.com/1103717Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567966}