Pass around CSSSelector by reference instead of pointer

Pass around CSSSelector by reference instead of pointer as it is expected
to be non-null. This makes the code look safer and gets rid of unnecessary
null-checks.

R=adamk

Review URL: https://codereview.chromium.org/149513011

git-svn-id: svn://svn.chromium.org/blink/trunk@166101 bbb929c8-8fbe-4397-9dbb-9b2b20218538