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Initial implementation of ephemeral tab based on overlay panel.
See go/ephemeral for the design.

Final design may be subject to change. and there are many TODO's
still left.

    non-trivial amount of time trying to reduce its number of methods.

Bug: 894619
Change-Id: I2833c6aaf820d9cf40be4d465ef135f5d5f8a9f6
Binary-Size: Increase needed to implement a feature, and I've already spent a
Reviewed-on: https://chromium-review.googlesource.com/c/1282043
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610325}