Add feature flag for single-tab mode and disable it by default.

Bug: 841643
Change-Id: I28d6ec0af4aa8ad77eeb9e140a3dac58097002ea
Reviewed-on: https://chromium-review.googlesource.com/1136064
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575375}
7 files changed