Web MIDI: fix ASSERT failures on WebPrivatePtr of WebMIDIPermissionRequest

BUG=256226

Review URL: https://chromiumcodereview.appspot.com/21546002

git-svn-id: svn://svn.chromium.org/blink/trunk@155394 bbb929c8-8fbe-4397-9dbb-9b2b20218538