Have SelectorQuery constructor take a Document reference in argument

Have SelectorQuery constructor take a Document reference in argument instead
of a pointer since it can never be NULL. This also avoids converting a
reference into a pointer at the call sites.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23851002

git-svn-id: svn://svn.chromium.org/blink/trunk@157049 bbb929c8-8fbe-4397-9dbb-9b2b20218538