We should unify various forms of tree traversal in DOM

Unifies tree-traversal methods in newer WTF::TreeNode class to reduce code
copying and ensure that traversals are as fast as possible.  This is hopefully
a first step towards unifying the data structures and traversal algorithms for
Node/ContainerNode and RenderObject/RenderObjectChildList.

The plan is to eventually merge NodeTraversal.h into TreeNode.h and replace all
usage of the former with the latter.  This first step is more modest in scope,
replacing only those methods that are templated and/or are copy pasted copies
between the current user of TreeNode.h -- HTMLImport -- and NodeTraversal.h.

BUG=331880

Review URL: https://codereview.chromium.org/125503002

git-svn-id: svn://svn.chromium.org/blink/trunk@164625 bbb929c8-8fbe-4397-9dbb-9b2b20218538