Fix deadlock when creating a frame cryptor on iOS (#157)

Co-authored-by: cloudwebrtc <[email protected]>

Author: davidliu

sdk/objc/api/peerconnection/RTCFrameCryptor.mm

@@ -138,8 +138,8 @@ - (nullable instancetype)initWithFactory:(RTC_OBJC_TYPE(RTCPeerConnectionFactory
             factory.signalingThread, [participantId stdString], mediaType,
             [self algorithmFromEnum:algorithm], keyProvider.nativeKeyProvider));
 
-    factory.workerThread->BlockingCall([self, nativeRtpSender] {
-      // Must be called on Worker thread
+    factory.signalingThread->BlockingCall([self, nativeRtpSender] {
+      // Must be called on signal thread
       nativeRtpSender->SetEncoderToPacketizerFrameTransformer(_frame_crypto_transformer);
     });
 
@@ -178,8 +178,8 @@ - (nullable instancetype)initWithFactory:(RTC_OBJC_TYPE(RTCPeerConnectionFactory
             factory.signalingThread, [participantId stdString], mediaType,
             [self algorithmFromEnum:algorithm], keyProvider.nativeKeyProvider));
 
-    factory.workerThread->BlockingCall([self, nativeRtpReceiver] {
-      // Must be called on Worker thread
+    factory.signalingThread->BlockingCall([self, nativeRtpReceiver] {
+      // Must be called on signal thread
       nativeRtpReceiver->SetDepacketizerToDecoderFrameTransformer(_frame_crypto_transformer);
     });