Introduce MIDIInputMap and MIDIOutputMap.

Currently MIDIAccess.inputs is a function that returns a sequence. With this CL
it will be an attribute of type MIDIInputMap, as specified in the spec.
MIDIAccess.outputs will be an attribute of type MIDIOutputMap in the same
manner. |forEach| is not yet supported.
The existing MIDIAccess.inputs and MIDIAccess.outputs functions cannot
be used with this CL.

BUG=341423

Review URL: https://codereview.chromium.org/513203002

git-svn-id: svn://svn.chromium.org/blink/trunk@181447 bbb929c8-8fbe-4397-9dbb-9b2b20218538