Enable Audio/Video dev monitoring and Video capture via AVFoundation by default.

Flipping the command line flag from explicitly enabling AVFoundation to
disable AVFoundation.

In the past there was a startup performance regression (http://crbug.com/311325 
and http://crbug.com/311437), those type of regression should be monitored 
carefully during the next couple of days.

BUG=288562

Review URL: https://codereview.chromium.org/101753003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239758 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239758