[DevTools] Check if focus on input element on copy command in elements panel.

If input element is focused we should proceed with system copy command handling.

BUG=263409

Review URL: https://chromiumcodereview.appspot.com/21928003

git-svn-id: svn://svn.chromium.org/blink/trunk@155618 bbb929c8-8fbe-4397-9dbb-9b2b20218538