Web MIDI: minimum support for Linux and Chrome OS (reland)

This change enables Web MIDI API on Linux and Chrome OS.
Only Sending functionality works with this first experimental change.

BUG=303601
TBR=scherkus@chromium.org
NOTRY=true

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=241524

Review URL: https://codereview.chromium.org/111443005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242871 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242871