Revert "Fix nullptr scrollbar crash"

This reverts commit 21576dbba343f81c6b7c02221ffa11542ffbae72.

Reason for revert: Crashes continued with same stack - ruling out the hypothesis.

Original change's description:
> Fix nullptr scrollbar crash
> 
> Not sure how this can happen. ScrollbarsFor somehow returns a nullptr in
> the ScrollbarSet. This must mean ToScrollbarLayer returns nullptr. This
> method returns nullptr in the base and this in the
> ScrollbarLayerImplBase override. Therefore the LayerToId muse be
> returning a non-scrollbar layer.
> 
> The only time we add anything to element_id_to_scrollbar_layer_ids_
> though is from LTI::RegisterScrollbar which is necessarily a scrollbar
> layer. I think this means the Layer with the scrollbar layer's ID at
> some point changes to a different layer. The ID-to-Layer mapping is
> cleared when a layer is destroyed and the scrollbar layer clears the
> element_id_to_scrollbar_layer_ids_ mapping in its destructor so I'm
> speculating that we might be registering a new non-scrollbar layer with
> the same id. I've converted the DCHECK to a CHECK at that location to
> see if it comes back with any crashes from the wild.
> 
> Bug: 924068
> Change-Id: I9bb15812aeda0a2c92d2547a52af9fbdbacc65b0
> Reviewed-on: https://chromium-review.googlesource.com/c/1435686
> Reviewed-by: Robert Flack <flackr@chromium.org>
> Reviewed-by: David Bokan <bokan@chromium.org>
> Commit-Queue: David Bokan <bokan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#630059}

TBR=flackr@chromium.org,bokan@chromium.org

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

Bug: 924068
Change-Id: I1fa905230e59b252ae7c63fe600bbfa22dec21f8
Reviewed-on: https://chromium-review.googlesource.com/c/1469284
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631508}
1 file changed