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This change enables incognito tab spawning and closing of all incognito
tabs from the new overflow menu.

BUG=817614

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_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: I0d5348ad2cbf740259d8cbd77be4437d59c480c4
Reviewed-on: https://chromium-review.googlesource.com/972156
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544847}