AVFoundation: Allow exceptions when checking if max frame rate setting is supported.

Addressing some sporadic crashes, see http://crbug.com/327532, with 
"Selector not found" when trying to query the AVCaptureConnection if 
it supports setting max video frame rate, at [1].

Beyond that, this problem started affecting all non-HD cameras in Mac
Os X < 10.9 -- see log in http://crbug.com/328096--.

This CL checks if the selector is recognised before attemping to use it.

[1] https://code.google.com/p/chromium/codesearch#chromium/src/media/video/capture/mac/video_capture_device_avfoundation_mac.mm&q=isvideomaxframe&sq=package:chromium&type=cs&l=174
BUG=288562, 327532, 328096

Review URL: https://codereview.chromium.org/99353005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241056 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241056