Web MIDI: Add a run-time flag

The flag is disabled by default for now.
--enable-web-midi command line flag enables a run-time flag for Web MIDI .
But it doesn't work for now.

BUG=163795
TEST=none

Review URL: https://codereview.chromium.org/14241011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@195766 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed