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Fuchsia SDK doesn't yet provide access to Clipboard, so this is just
a dummy implementation to make it possible to compile ui/base

Bug: 740608
Change-Id: I3add17aad177787d2c7ee0f639ac9fc08286a54d
Reviewed-on: https://chromium-review.googlesource.com/565220Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485483}