DevTools: Move platform detection from the "ui" into the "common" module

Platform detection is also used by non-ui pieces of the frontend

R=aandrey, eustas, vsevik

Review URL: https://codereview.chromium.org/137793014

git-svn-id: svn://svn.chromium.org/blink/trunk@165240 bbb929c8-8fbe-4397-9dbb-9b2b20218538