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This change is in preparation of multipaste related VK API changes,
where the ChromeVirtualKeyboardDelegate will need to be a
ClipboardHistoryControllerObserver. This will allow the VK to observe
clipboard history changes.

Bug: 1099318
Change-Id: I5535d45255b137219cf76b76f6d94c9bfb5ca9e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2573636Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833831}