Add additional test to verify that setMediaKeys() can be called before setting .src on HTMLMediaElement.

BUG=349546
TEST=new layout test passes

Review URL: https://codereview.chromium.org/189583002

git-svn-id: svn://svn.chromium.org/blink/trunk@168863 bbb929c8-8fbe-4397-9dbb-9b2b20218538