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This is a reland of 640f197a

Original change's description:
> Reland: Various refactoring in TabStripModel tests.
>
> This patch refactors tab_strip_model_unittest.cc and some associated
> files in several ways:
> * Removes ChromeRenderViewHostTestHarness as a superclass of
>   TabStripModelTest, which includes some heavyweight dependencies that
>   are unused. This speeds up the suite by about 20% (~6500 ms runtime
>   vs. ~8000ms before this patch).
> * Changes TestTabStripModelDelegate::RunUnloadListenerBeforeClosing to
>   return false, and deletes subclasses that exist solely to override
>   that method (all of them, except TabStripDummyDelegate, which is
>   renamed to UnloadListenerTabStripModelDelegate).
> * Moved several helper classes to be defined immediately before the
>   test that uses them.
> * Deletes member variables from MockTabStripModelObserver, which are
>   unused.
> * Fixes lint errors and cleans up unused includes.
>
> Change-Id: Ibb93ff41a21aab4c0b9b9f075df315381959c35c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538575
> Reviewed-by: François Doray <fdoray@chromium.org>
> Reviewed-by: Erik Chen <erikchen@chromium.org>
> Commit-Queue: Bret Sepulveda <bsep@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#644437}

TBR=erikchen@chromium.org,fdoray@chromium.org

Change-Id: I1ed5fbe5cafab20271be5148596f6fed9e493b73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1540261Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644536}