Convert editing/pasteboard/5075944.html with assert_selection

This patch converts editing/pasteboard/5075944.html with assert_selection
to promote the use of w3c testharness and improve code health.

It is also a preparation for https://codereview.chromium.org/2626933002

BUG=679616, 679977

Review-Url: https://codereview.chromium.org/2621213002
Cr-Commit-Position: refs/heads/master@{#442820}