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This change adds an initial, non-functional overflow menu.  It can be
summoned, looks like an overflow menu, and has buttons and items.  It
does not yet:

- Trigger any actions on clicks
- Appear when hovering the overflow button
- Update according to tab state
- Expand to fit translated text

These additions will come in follow-on changes.

BUG=817614

Cq-Include-Trybots: luci.chromium.try:linux_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I837f7a3b4cb0b151e9370ba3d7e2f2de00bd34bc
Reviewed-on: https://chromium-review.googlesource.com/967256
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544087}