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Some constants in ash_constants.h are used in
BrowserNonClientFrameViewAsh. In order to make this
class available to lacros, these constants need to be
factored out, once lacros chrome can not link to ash symbols.

The following constants are moved:

 ash::kDefaultFrameColor
 ash::kResizeAreaCornerSize
 ash::kResizeInsideBoundsSize
 ash::kResizeOutsideBoundsSize
 ash::kResizeOutsideBoundsScaleForTouch

now under the `chromeos` namespace.

Also, CL adds a README.md to chromeos/ui directory.

This is phase 2.2 on the design document [1].

[1] https://docs.google.com/document/d/1xHwcHrAiEaWuA4usGEqKZ9zm1H8SGk3WkS-Jf2Q_los/

BUG=1113900
R=jamescook@chromium.org

Change-Id: I01cf20ed5a77f752fee2c72df0d5cb0cf42125a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429441Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Antonio Gomes (GMT-4) <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#811027}