Change EME content browser tests to use W3C test harness

There are a couple of EME tests that aren't using the W3C test harness.
In preparation for converting them to use promises, update the tests
to use the W3C test harness.

TEST=modified tests pass

Review URL: https://codereview.chromium.org/274243003

git-svn-id: svn://svn.chromium.org/blink/trunk@174064 bbb929c8-8fbe-4397-9dbb-9b2b20218538