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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/+/2346338Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
Cr-Commit-Position: refs/heads/master@{#797572}