DevTools: Extract RootView into "ui"

This is required to eliminate the WI.Toolbox -> "components" dependency

R=dgozman, vsevik
BUG=391566

Review URL: https://codereview.chromium.org/441423003

git-svn-id: svn://svn.chromium.org/blink/trunk@179722 bbb929c8-8fbe-4397-9dbb-9b2b20218538