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This CL also adds no-op functions to return supported overlay
coordinator classes.

Bug: 1030357
Change-Id: I77c48fd1833509044a8c440d45e7f82218f06ebc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949288
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721605}