Web MIDI: make testRunner be able to control startSession result

TEST=LayoutTests/webmidi
BUG=260315

Review URL: https://chromiumcodereview.appspot.com/23093021

git-svn-id: svn://svn.chromium.org/blink/trunk@156634 bbb929c8-8fbe-4397-9dbb-9b2b20218538