Web MIDI: remove deprecated API, void WebMIDIAccessorClient::didStartSession()

This is the last CL of a three step API update.

FYI;
 1. https://codereview.chromium.org/23376004/
 2. https://codereview.chromium.org/23021018/

TEST=LayoutTests/webmidi
BUG=260315

Review URL: https://chromiumcodereview.appspot.com/22909049

git-svn-id: svn://svn.chromium.org/blink/trunk@156676 bbb929c8-8fbe-4397-9dbb-9b2b20218538