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NavigatorClipboard is mostly boilerplate, and Clipboard can be the
Supplement<Navigator> cleanly.

Change-Id: Ieb7db5f8172a08c193dff261bcaa43fda7f47dd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2499367
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Auto-Submit: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821812}