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The base Clipboard interface supports different types of clipboards, such as
primary selection on X11 or drag on OS X.  Ozone didn't support this concept
at all, which made it impossible to use more than one clipboard.

This CL adds support of multiple clipboards at the Ozone level and implements
the selection clipboard for Ozone/X11.

R=dcheng@chromium.org, msisov@igalia.com, rjkroege@chromium.org

Bug: 973295
Change-Id: I05ed4aa93517d80e1c789a2a09d249a2b923546d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1781142Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Maksim Sisov <msisov@igalia.com>
Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
Cr-Commit-Position: refs/heads/master@{#695913}