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This is a reland of 6eb61402

Original change's description:
> Removes ui/platform_window/mojo as it is not used by anyone.
> 
> This is to remove the ref to some types under ui/base/ime/mojo, which will be removed later.
> 
> Bug: 965908
> Change-Id: I7979524066104c829b4f4e287a0ab87c9365f124
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626055
> Reviewed-by: Dominick Ng <dominickn@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Shu Chen <shuchen@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#662924}

Bug: 965908
Change-Id: I6bb1c6d081ce235c959cc30e53d20b57dd0e1459
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626841Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Shu Chen <shuchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663381}