Handle iOS devices with no rear-facing camera

Devices exist (specifically the 16GB 5th generation iPod Touch) that do not have
a rear-facing camera.

This CL:
- Adjusts RTCAVFoundationVideoCapturerInternal so initialization doesn't fail
because a rear-facing camera doesn't exist, but still logs a warning
- Provides a check for whether or not a rear-facing camera can be used (useful
for toggling UI elements)
- Turns an attempt to switch to the rear-facing camera into a no-op with a
warning.

BUG=

Review URL: https://codereview.webrtc.org/1416303003

Cr-Commit-Position: refs/heads/master@{#11992}
diff --git a/webrtc/api/objc/RTCAVFoundationVideoSource.mm b/webrtc/api/objc/RTCAVFoundationVideoSource.mm
index 0cd0155..1005c7d 100644
--- a/webrtc/api/objc/RTCAVFoundationVideoSource.mm
+++ b/webrtc/api/objc/RTCAVFoundationVideoSource.mm
@@ -27,6 +27,10 @@
   return [super initWithNativeVideoSource:source];
 }
 
+- (BOOL)canUseBackCamera {
+  return self.capturer->CanUseBackCamera();
+}
+
 - (BOOL)useBackCamera {
   return self.capturer->GetUseBackCamera();
 }