Convert execCommand/unlink.html to use w3c test harness

This patch converts "execCommand/unlink.html" to use w3c test
harness with |assert_selection()| to simplify for code and improve code health.

This patch also updates |assert_selection()| to serialize selection correctly
when anchor node and focus node aren't |Text| node, and adds sanity check
for expected text.

This patch is a preparation of committing, Make
FrameSelection to track associated Document.

TEST=n/a; no behavior changes

Cr-Commit-Position: refs/heads/master@{#396791}
3 files changed