Web MIDI: enable important logs in release build

To investigate a reason that Web MIDI goes not working recently in ChromeOS,
this change enable some important log messages in release build.
Also, this change make it impossible to disable MIDIManager for ChromeOS.

BUG=335865

Review URL: https://codereview.chromium.org/166033004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251564 0039d316-1c4b-4281-b951-d872f2087c98