Convert editing/pasteboard/8145-[1-3].html to use w3c test harness

This patch converts "editing/pasteboard/8145-[1-3].html" to use w3c
test harness and consolidates into "insert_html_as_plain_text.html" to simplify
test case for ease of maintenance.

This patch is a preparation of getting rid of grammar checking feature.

BUG=619452
TEST=n/a; this patch changes a test file

Review-Url: https://codereview.chromium.org/2179373002
Cr-Commit-Position: refs/heads/master@{#407735}
24 files changed