fix the crash in QTMonitorImpl::Start()

It seems QTMonitorImpl::Start() crashed when getting the initial number of input devices in some conner cases.

crash log:
CMIOUnitCreateFromDescription
CMIOUnitCreateFromDescription
+[QTCaptureVDIGDevice _refreshDevices]
+[QTCaptureVDIGDevice devicesWithIOType:]
+[QTCaptureDevice devicesWithIOType:]
+[QTCaptureDevice inputDevices]
device_monitor_mac.mm:43]	content::DeviceMonitorMac::QTMonitorImpl::Start

I guess it is because some racing between chrome is launched and we enumerate the devices in QTMonitorImpl::Start().
A quick fix will be not to enumerate the devices in Start(), though this means that we will have to send one more device changed notification when a device is changed at the first time.

BUG=162324
TEST=manual go to content settings and look at the the device lists.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=169285

[Avi (use Gerrit), 2012-11-22 14:46:46]

:( lgtm

[no longer working on chromium, 2012-11-22 15:08:35]

Thanks, I am not sure if this is completely safe, the stability of the device
monitor is very important and I will keep monitoring the go/crash.

[commit-bot: I haz the power, 2012-11-22 15:08:44]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/xians@chromium.org/11414127/1

[commit-bot: I haz the power, 2012-11-22 17:26:00]

Change committed as 169285