Have SelectorQuery API take rootNode by reference

Have SelectorQuery API take rootNode by reference instead of taking a pointer
as it is expected to be non-null. This also gets rid of several unnecessary
null checks.

R=adamk

Review URL: https://codereview.chromium.org/53683007

git-svn-id: svn://svn.chromium.org/blink/trunk@161095 bbb929c8-8fbe-4397-9dbb-9b2b20218538