[DevTools] Add Page.canEmulate capability.

This will allow to disable emulation on targets which don't need it, for example DevTools itself.

BUG=none

Review URL: https://codereview.chromium.org/501343002

git-svn-id: svn://svn.chromium.org/blink/trunk@180953 bbb929c8-8fbe-4397-9dbb-9b2b20218538