Rename nodes/elements traversal function names.

Rename the following functions:

  NodeTraversal::startsFrom(const Node*) (and Traversal<ElementType>::startsFrom(const ElementType*)
  NodeTraversal::startsFromNextOf(const Node&) (and Traversal<ElementType>::startsFromNextOf(const ElementType&)

to:

  NodeTraversal::startsAt(const Node*) (and Traversal<ElementType>::startsAt(const ElementType*)
  NodeTraversal::startsAfter(const Node&) (and Traversal<ElementType>::startsAfter(const ElementType&)

so that they have more meaningful names.

This patch also removes an unnecessary underlying iterator, TraversalInclusiveNextIterator.

BUG=None
TEST=no layout test failure

Review URL: https://codereview.chromium.org/667403002

git-svn-id: svn://svn.chromium.org/blink/trunk@184287 bbb929c8-8fbe-4397-9dbb-9b2b20218538