Fixing warnings in public iOS SDK headers.

Building with the newly published cocoapod generated a few warnings,
which looked a bit bad.

Bug: webrtc:8831
Change-Id: I70c06930603b328e4d11c599a5b5dd77b45150c6
Reviewed-on: https://webrtc-review.googlesource.com/46163
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Peter Hanspers <peterhanspers@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21846}

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

@@ -168,7 +168,7 @@ const int64_t kNanosecondsPerSecond = 1000000000;
                     }];
 }
 
-- (void)stopCaptureWithCompletionHandler:(nullable void (^)())completionHandler {
+- (void)stopCaptureWithCompletionHandler:(nullable void (^)(void))completionHandler {
   _willBeRunning = NO;
   [RTCDispatcher
       dispatchAsyncOnType:RTCDispatcherTypeCaptureSession

sdk/objc/Framework/Classes/PeerConnection/RTCMetrics.mm

@@ -12,11 +12,11 @@
 
 #import "RTCMetricsSampleInfo+Private.h"
 
-void RTCEnableMetrics() {
+void RTCEnableMetrics(void) {
   webrtc::metrics::Enable();
 }
 
-NSArray<RTCMetricsSampleInfo *> *RTCGetAndResetMetrics() {
+NSArray<RTCMetricsSampleInfo *> *RTCGetAndResetMetrics(void) {
   std::map<std::string, std::unique_ptr<webrtc::metrics::SampleInfo>>
       histograms;
   webrtc::metrics::GetAndReset(&histograms);

sdk/objc/Framework/Classes/PeerConnection/RTCSSLAdapter.mm

@@ -13,13 +13,13 @@
 #include "rtc_base/checks.h"
 #include "rtc_base/ssladapter.h"
 
-BOOL RTCInitializeSSL() {
+BOOL RTCInitializeSSL(void) {
   BOOL initialized = rtc::InitializeSSL();
   RTC_DCHECK(initialized);
   return initialized;
 }
 
-BOOL RTCCleanupSSL() {
+BOOL RTCCleanupSSL(void) {
   BOOL cleanedUp = rtc::CleanupSSL();
   RTC_DCHECK(cleanedUp);
   return cleanedUp;

sdk/objc/Framework/Classes/PeerConnection/RTCTracing.mm

@@ -12,7 +12,7 @@
 
 #include "rtc_base/event_tracer.h"
 
-void RTCSetupInternalTracer() {
+void RTCSetupInternalTracer(void) {
   rtc::tracing::SetupInternalTracer();
 }
 
@@ -20,10 +20,10 @@ BOOL RTCStartInternalCapture(NSString *filePath) {
   return rtc::tracing::StartInternalCapture(filePath.UTF8String);
 }
 
-void RTCStopInternalCapture() {
+void RTCStopInternalCapture(void) {
   rtc::tracing::StopInternalCapture();
 }
 
-void RTCShutdownInternalTracer() {
+void RTCShutdownInternalTracer(void) {
   rtc::tracing::ShutdownInternalTracer();
 }