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History: AVFoundation and QTKit are the two options for audio/video 
monitoring and video capture in Mac. AVFoundation is currently enabled
in Mac OS X >= 10.7, and only via command line flag, which is currently
enabled in developer and Canary builds. 

This CL adds a --force-qtkit flag that suppresses AVFoundation usage
for those versions and channels that otherwise would use it.

Please note the naming: --force-qtkit vs. --enable-avfoundation

BUG=288562, 369024, 357250

Review URL: https://codereview.chromium.org/260873011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268065 0039d316-1c4b-4281-b951-d872f2087c98