[DevTools] Disable emulation UI elements when emulation itself is disabled.

This was broken because we have two conditions to check:
- whether inspected page is on real device;
- whether emulation is enabled.
Second condition was ignored.

This change also applies emulation on first click on 'Emulate screen' checkbox.

BUG=330504

Review URL: https://codereview.chromium.org/118513004

git-svn-id: svn://svn.chromium.org/blink/trunk@164328 bbb929c8-8fbe-4397-9dbb-9b2b20218538