Web MIDI: move MIDI message validating logic into message_util.{cc|h}

There are two motivations for this changes:

 1. To reduce duplicated message handling code
 2. To simplify MidiHost to make coming mojofication change as much
    simple as possible

Also this change rename midi_message_util to message_util
since now that midi has a dedicated namespace.

BUG=339749, 582327

Review-Url: https://chromiumcodereview.appspot.com/2431393002
Cr-Commit-Position: refs/heads/master@{#426454}
13 files changed