Added AVFoundation Glue and Device Monitoring for Mac.

This CL adds files called avfoundation_glue.{h,mm} to media/video/capture/mac.
These files encapsulate dynamic loading of AVFoundation libraries and define+
implement a facade to the used AVFoundation classes. The first usage of those
is the DeviceMonitorMac, where the original QTKit event class is transformed
into an interface (MacMonitorInterface), implemented by both QTKitMonitorImpl
and AVFoundationMonitorImpl. the usage of one or the other is decided on
DeviceMonitorMac constructor time.

This CL is part of a larger exercise to add support for Video Capture in
Mac > 10.6 using AVFoundation.

BUG=288562

Review URL: https://codereview.chromium.org/24615005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230553 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230553