Web MIDI: add assign() and equals() to WebMIDIPermissionRequest

Since WebMIDIPermissionRequeset should be managed by embedder
until the permission decision is made, export assign() and
equals() methods. It makes it possible to handle them in a
container.

TEST=none
BUG=163795

Review URL: https://chromiumcodereview.appspot.com/18563008

git-svn-id: svn://svn.chromium.org/blink/trunk@154188 bbb929c8-8fbe-4397-9dbb-9b2b20218538