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If the clipboard contains data that has been marked by its originator
as confidential, don't offer it for "Paste and Search" and don't offer
it as a zero-prefix search suggestion.

This directly wires into the iOS zero-prefix search code, and adds an
expandable function to ui::Clipboard, implementing it for the Mac.

If there is a convention for other platforms, it can be implemented
via this mechanism.

Bug: 1067542, 1084244, 1071828
Change-Id: I064a0629c35b686e14772cec123a3fc1eec5487c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2213286Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771982}