Move core/platform/midi to modules/webmidi

There doesn't seem to be any reason for this code to be in the platform
directory. We might as well move it to the webmidi module next to the code that
uses it.

R=eseidel
BUG=297477

Review URL: https://codereview.chromium.org/25722004

git-svn-id: svn://svn.chromium.org/blink/trunk@158753 bbb929c8-8fbe-4397-9dbb-9b2b20218538