[import] Make the import tree linked-list based.

This change introdues TreeNode class, which implements
ContainerNode-like generic intrusive tree data structure,
and replaces array-based tree implementation on HTMLImport with TreeNode.

Treely is better than array-based one when it comes tree traversal:
It supports iterative DFS. And coming changes need it.

BUG=319151
R=hayato,tasak,esprehn
TEST=TreelyTest.cpp

Review URL: https://codereview.chromium.org/77883002

git-svn-id: svn://svn.chromium.org/blink/trunk@162353 bbb929c8-8fbe-4397-9dbb-9b2b20218538