Fix building RTCCameraVideoCapturereTests with iOS 11 SDK.

The iOS 11 SDK adds nullability annotations to several framework functions
and in this it added the _Nonnull specifier to a protocol method that
we implement. We were passing nil to that method in a test.
The warning is now fixed by passing a mock object instead of nil.

Bug: webrtc:7883
Change-Id: I9f64b0c59750629ca3969400aa725729bb10541b
Reviewed-on: https://chromium-review.googlesource.com/549927
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Kári Tristan Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18780}

webrtc/sdk/objc/Framework/UnitTests/RTCCameraVideoCapturerTests.mm

@@ -64,16 +64,19 @@ CMSampleBufferRef createTestSampleBufferRef() {
 @interface RTCCameraVideoCapturerTests : NSObject
 @property(nonatomic, strong) id delegateMock;
 @property(nonatomic, strong) id deviceMock;
+@property(nonatomic, strong) id captureConnectionMock;
 @property(nonatomic, strong) RTCCameraVideoCapturer *capturer;
 @end
 
 @implementation RTCCameraVideoCapturerTests
 @synthesize delegateMock = _delegateMock;
+@synthesize captureConnectionMock = _captureConnectionMock;
 @synthesize capturer = _capturer;
 @synthesize deviceMock = _deviceMock;
 
 - (void)setup {
   self.delegateMock = OCMProtocolMock(@protocol(RTCVideoCapturerDelegate));
+  self.captureConnectionMock = OCMClassMock([AVCaptureConnection class]);
   self.capturer = [[RTCCameraVideoCapturer alloc] initWithDelegate:self.delegateMock];
   self.deviceMock = [self createDeviceMock];
 }
@@ -169,7 +172,7 @@ CMSampleBufferRef createTestSampleBufferRef() {
   // when
   [self.capturer captureOutput:self.capturer.captureSession.outputs[0]
          didOutputSampleBuffer:sampleBuffer
-                fromConnection:nil];
+                fromConnection:self.captureConnectionMock];
 
   // then
   [self.delegateMock verify];
@@ -203,7 +206,7 @@ CMSampleBufferRef createTestSampleBufferRef() {
 
   [self.capturer captureOutput:self.capturer.captureSession.outputs[0]
          didOutputSampleBuffer:sampleBuffer
-                fromConnection:nil];
+                fromConnection:self.captureConnectionMock];
 
   [self.delegateMock verify];