[DevTools] More precise control over emulation params.

This change allows to emulate only viewport and device scale factor,
for example, leaving everything else untouched.

BUG=327641

Review URL: https://codereview.chromium.org/271673003

git-svn-id: svn://svn.chromium.org/blink/trunk@173869 bbb929c8-8fbe-4397-9dbb-9b2b20218538