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This reverts commit c756c75c.

Reason for revert: Suspected cause of crbug.com/1142873

Original change's description:
> [iOS] Fix tab strip views when using FullscreenProvider
>
> When the FullscreenProvider is used, the layout of the browser views is
> slightly different. Instead of having the web content views start from
> the bottom of the toolbar, the web content extends all the way to the
> top of the BrowserContainerViewController, behind the tab strip.
>
> This means that the prior approach of making all the top background
> views clear does not work when using FullscreenProvider. Instead, this
> CL uses the following approach: set these background views to have
> the same background color as the tab grid. As soon as the panning
> gesture starts, make them clear so the tab grid shows through and the
> animations appear correctly. At the same time, move the web content
> views down so they don't obstruct the animations either.
>
> Bug: 1094335
> Change-Id: Ieea7d268e781da5e1ccd4c2c0ad3c0925407d133
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489898
> Reviewed-by: Mark Cogan <marq@chromium.org>
> Commit-Queue: Robbie Gibson <rkgibson@google.com>
> Cr-Commit-Position: refs/heads/master@{#821200}

TBR=marq@chromium.org,rkgibson@google.com,mouraroberto@google.com

Change-Id: I2b026d35000d34b29de9235ed2752273f6f7dd33
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1142873
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2502574Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821278}