Fix NSInteger formatting warning from clang 6

"error: values of type 'NSInteger' should not be used as format arguments; add an explicit cast to 'long' instead"
Casting to long is already a common practice in the code base.

This has been blocking the Chromium roll which contains an update to clang 6.0.0

BUG=None

Review-Url: https://codereview.webrtc.org/2987693002
Cr-Commit-Position: refs/heads/master@{#19127}

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCCameraVideoCapturer.m

@@ -121,7 +121,7 @@ static inline BOOL IsMediaSubTypeSupported(FourCharCode mediaSubType) {
   [RTCDispatcher
       dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                     block:^{
-                      RTCLogInfo("startCaptureWithDevice %@ @ %zd fps", format, fps);
+                      RTCLogInfo("startCaptureWithDevice %@ @ %ld fps", format, (long)fps);
 
 #if TARGET_OS_IPHONE
                       [[UIDevice currentDevice] beginGeneratingDeviceOrientationNotifications];