Port ParentNode API to ElementTraversal

Port ParentNode API to ElementTraversal and drop corresponding API on
ContainerNode. All callers of the ContainerNode API were updated to use
ElementTraversal directly. This is more consistent and avoid inefficiencies
due to callers assuming those API methods are simple getters although they
need to traverse the DOM tree. It is now obvious to callers that they are
traversing the DOM tree and that they should cache the return value.

R=adamk, eseidel

Review URL: https://codereview.chromium.org/166573003

git-svn-id: svn://svn.chromium.org/blink/trunk@167226 bbb929c8-8fbe-4397-9dbb-9b2b20218538