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This is a reland of e4a4b7a3

Fixes null dereference bug

Original change's description:
> Replace ThumbnailImage::Observer with Subscriber
>
> Previously, clients interested in seeing thumbnail updates had to
> derive from ThumbnailImage::Observer and register themselves with
> ThumbnailImage.
>
> Removing ThumbnailTabHelper and ThumbnailImage from the public API and
> instead exposing a global ThumbnailService is a design
> goal. ThumbnailImage::Observer makes this refactor difficult.
>
> This CL uses a callback-based subscriber model instead.
> base::Callbacks replace observer methods. Hints are supplied
> directly on the subscription object. The subscription models loosens
> the coupling between clients and ThumbnailImage.
>
> Clients subscribe by ThumbnailImage::Subscribe(). Subscription can
> later be pushed up easily to the ThumbnailService level by a method
> like ThumbnailService::Subscribe(content::WebContents*).
>
> Bug: 1090038
> Change-Id: I020d848e00331d5f5bbb562f0c53aec5f8ccbc62
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593158
> Reviewed-by: Dana Fried <dfried@chromium.org>
> Reviewed-by: John Lee <johntlee@chromium.org>
> Commit-Queue: John Lee <johntlee@chromium.org>
> Auto-Submit: Collin Baker <collinbaker@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#837818}

Bug: 1090038
Change-Id: I3d93fc36d9725c088ba684188265df1ad1fd0905
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612046
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Reviewed-by: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842260}