Revert 239920 "Revert 239758 "Enable Audio/Video dev monitoring ..."

Reverting the revert. Congrats, the original CL wasn't the cause for the failure; sorry for the mess.

> Revert 239758 "Enable Audio/Video dev monitoring and Video captu..."
> 
> > 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
> 
> TBR=mcasas@chromium.org
> 
> Review URL: https://codereview.chromium.org/112033003

TBR=vadimt@chromium.org

Review URL: https://codereview.chromium.org/98333004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239939 0039d316-1c4b-4281-b951-d872f2087c98
[GCM] Add a working MCS client and probe tool.

The MCS client maintains the connection with the MCS endpoint and handles
all reliability issues. This is the meat and potatoes of the GCM logic.

The probe tool allows testing the MCS client against a production server using
real connections and arbitrary credentials.

BUG=284553

Review URL: https://codereview.chromium.org/89143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239940 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239940