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This is a reland of 1523bb86

Original change's description:
> [ozone] Improved clipboard tests and de-duplicated string constants.
> 
> This CL pays a bit of technical debt in the clipboard implementation.
> 
> First, it updates a test that verifies that putting text into clipboard
> registers the proper set of MIME types, as had been suggested in [1].
> 
> Second, it enables for Ozone the test that verifies selection buffers.
> 
> Third, it de-duplicates some string constants.
> 
> [1] https://chromium-review.googlesource.com/c/chromium/src/+/2306252
> 
> Change-Id: I6beb6b5b3827f0ec09cf19bc1868e7955141f680
> Bug: 911992, 1105892
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346338
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
> Cr-Commit-Position: refs/heads/master@{#797572}

Bug: 911992
Bug: 1105892
Change-Id: I832d56641910a7014d7b53a8ce19fd3c71be2c4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352803Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
Cr-Commit-Position: refs/heads/master@{#798019}