Convert editing/pasteboard/merge-end-blockquote.html to use w3c test harness

This patch converts "editing/pasteboard/merge-end-blockquote.html" to use w3c
test harness to simplify test files.

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/2181113003
Cr-Commit-Position: refs/heads/master@{#407760}
7 files changed