multipaste: Add flag for experimental Ctrl+V behavior

This CL introduces a flag that, when enabled, causes long-pressed
Ctrl+V to show the clipboard history menu.

Fixed: b/274155350
Change-Id: I8efbcac89b8b108c781320e2eb1d6fcf4b65887d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4347587


Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: Colin Kincaid <ckincaid@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118885}