Forward-declare grpc_channel and specify ownership semantics

src/objective-c/GRPCClient/private/GRPCChannel.h

@@ -33,12 +33,14 @@
 
 #import <Foundation/Foundation.h>
 
-#include <grpc/grpc.h>
+struct grpc_channel;
 
 // Each separate instance of this class represents at least one TCP connection to the provided host.
 // Create them using one of the subclasses |GRPCSecureChannel| and |GRPCUnsecuredChannel|.
 @interface GRPCChannel : NSObject
-@property(nonatomic, readonly) grpc_channel *unmanagedChannel;
+@property(nonatomic, readonly) struct grpc_channel *unmanagedChannel;
 
-- (instancetype)initWithChannel:(grpc_channel *)unmanagedChannel NS_DESIGNATED_INITIALIZER;
+// This initializer takes ownership of the passed channel, and will destroy it when this object is
+// deallocated. It's illegal to pass the same grpc_channel to two different GRPCChannel objects.
+- (instancetype)initWithChannel:(struct grpc_channel *)unmanagedChannel NS_DESIGNATED_INITIALIZER;
 @end

src/objective-c/GRPCClient/private/GRPCChannel.m

@@ -33,6 +33,8 @@
 
 #import "GRPCChannel.h"
 
+#include <grpc/grpc.h>
+
 @implementation GRPCChannel
 
 - (instancetype)init {