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Supports only CLIPBOARD selection and not PRIMARY (for now) since
ozone only supports a single clipboard.

Regisers to receive XFixesSetSelectionOwnerNotify events and
prefetches and caches supported mime types whenever ownership
changes.  Also prefetches clipboard contents as 'text/plain'
since that is the most likely type to be requested.  If clients
request a different type, it will be fetched at the time of
requesting.

Converts text/plain[;charset=utf-8] <=> [UTF8_]STRING to
allow interoperability with other applications which do not
use the same mime types as chrome.

This code can be patched into linux-chromeos (chrome os running
on a linux desktop) by modifying the build file:
 sed -i s/is_desktop_linux/is_linux/ ui/base/clipboard/BUILD.gn

Bug: 985187
Change-Id: I4277f903ab8f11e0f5ad14d095eb099cf775f771
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1736619Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691892}