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Chrome OS supports the concept of a "root window for new windows",
which allows new windows to be automatically placed on the correct
display. This knowledge lives in ash, currently inside the browser
process.

Remote processes that use views::Widget (e.g. the shortcut viewer app)
sometimes need to know what window they are going to open on,
for example to do initial bounds computations.

Change Screen to provide set and get for the display for new windows.
Use mojo to send the information to other processes via ScreenProvider
(on the ash side) and ScreenMus (on the remote side). Send the display
id as part of display updates so that ScreenMus gets it on startup,
and also so updates are atomic with other display changes.

Once this lands we'll be able to port browser windows to use this
mechanism and delete the ash::ShellState / chrome ShellStateClient
code I built as a one-off for browser windows under mash.

Bug: 867559, 764009, 826569
Test: added unit tests
Change-Id: I6b5beb2610243f30072c3d73180b3ce21a310ca6
Reviewed-on: https://chromium-review.googlesource.com/1162980Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581055}