Use stricter typing for traverse roots in SelectorQuery

Use stricter typing for traverse roots in SelectorQuery. We now use Element or
ContainerNode, as appropriate. This enables using the faster DOM traversal method
specializations as the compiler now knows these are container nodes and not
simply generic Node types.

R=esprehn, adamk

Review URL: https://codereview.chromium.org/153073002

git-svn-id: svn://svn.chromium.org/blink/trunk@166344 bbb929c8-8fbe-4397-9dbb-9b2b20218538