[DevTools] Introduced WebDeviceEmulationParams.

BUG=327641
TBR=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/222173002

git-svn-id: svn://svn.chromium.org/blink/trunk@170670 bbb929c8-8fbe-4397-9dbb-9b2b20218538