Move Node::retarget() to TreeScope::retarget()

It would be better to make TreeScope have this function.
This CL also changes the parameter type from Node to Element because
Element is only the use case, as of now.

BUG=531990

Review URL: https://codereview.chromium.org/1879373002

Cr-Commit-Position: refs/heads/master@{#387562}
4 files changed