Add a test to ensure NPP_URLNotify is called exactly once on plugin removal.

This is a regression test for https://codereview.chromium.org/223883003.

BUG=358516

Review URL: https://codereview.chromium.org/224013003

git-svn-id: svn://svn.chromium.org/blink/trunk@170883 bbb929c8-8fbe-4397-9dbb-9b2b20218538