Web MIDI: remove obsolete API

didStartSession(bool) is not obsolete and new version
didStartSession(bool, const blink::WebString&, const blink::WebString&)
should be used instead.

BUG=n/a
TEST=LayoutTests/webmidi

Review URL: https://codereview.chromium.org/263113002

git-svn-id: svn://svn.chromium.org/blink/trunk@173384 bbb929c8-8fbe-4397-9dbb-9b2b20218538