Adds a 'Close' button to the custom tab bar view.

The close button navigates back to the last in scope url, if any. If no
in scope url is available (as when using window.location.replace(...))
the app's launch_url is navigated to and the history stack is cleared.

Notes:
- In future, the forward stack should be cleared when the close button
is clicked.

Screenshot:
https://bugs.chromium.org/p/chromium/issues/attachment?aid=372336&signed_aid=Zqm633OoKgTghf6XijsROQ==&inline=1

Bug: 853593
Change-Id: I0868500e6a98445ed5113b61bb6a7529bccb7d2c
Reviewed-on: https://chromium-review.googlesource.com/c/1381391
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Jay Harris <harrisjay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617736}
4 files changed