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This is a reland of 2a23cb68

The build failure was due to an ash header being included in a
non-ChromeOS build. This managed to build correctly until my CL.
The reland makes the include conditional on building for ChromeOS.

TBR=oshima@chromium.org

Original change's description:
> Scale source window during tab drag-and-drop
>
> This behavior is very similar to tablet mode tab drags from the native
> tab strip. Some of this code is copied, and some is shared (through
> TabletModeBrowserWindowDragSessionWindowsHider).
>
> Bug: 1069869
> Change-Id: If5c6f163f7d8af9918be3bc97ca8a91b1df063c1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2202507
> Commit-Queue: Collin Baker <collinbaker@chromium.org>
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#770272}

Bug: 1069869
Change-Id: I02f6becf72fac663a744d2033e16e2fd14dce843
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209384Reviewed-by: Collin Baker <collinbaker@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770322}