Port SiblingTraversalStrategies to ElementTraversal API

Port SiblingTraversalStrategies to ElementTraversal API for consistency.
Node's previousElementSibling() / nextElementSibling() will be wrappers around
ElementTraversal soon.

Also pass the element by reference instead of pointer in
SiblingTraversalStrategies's API to make the code cleaner.

R=adamk, eseidel

Review URL: https://codereview.chromium.org/167013002

git-svn-id: svn://svn.chromium.org/blink/trunk@167223 bbb929c8-8fbe-4397-9dbb-9b2b20218538