Export only Cronet API symbols and hide all others.

components/cronet/ios/Cronet.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import <Foundation/Foundation.h>
 
 #include "cronet_c_for_grpc.h"
 
 // Interface for installing Cronet.
+CRONET_EXPORT
 @interface Cronet : NSObject
 
 // Sets whether HTTP/2 should be supported by CronetEngine. This method only has
 // any effect before |start| is called.
 + (void)setHttp2Enabled:(BOOL)http2Enabled;
 
 // Sets whether QUIC should be supported by CronetEngine. This method only has
 // any effect before |start| is called.
 + (void)setQuicEnabled:(BOOL)quicEnabled;
 
@@ skipping 26 matching lines @@
 + (void)stopNetLog;
 
 // Returns the full user-agent that the stack uses.
 // This is the exact string servers will see.
 + (NSString*)getUserAgent;
 
 // Get a pointer to global instance of cronet_engine for GRPC C API.
 + (cronet_engine*)getGlobalEngine;
 
 @end