Add paste menu item to the gear menu.

Test: browser_tests --gtest_filter=GearMenu*
Bug: 796815
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Iab214e1e4e991518e8c8c02a37ebd7a5c93e93f3
Reviewed-on: https://chromium-review.googlesource.com/841962
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531484}