[Cronet] Export NSError constants instead of defining them in Cronet.h

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 "bidirectional_stream_c.h"
 
 // Type of HTTP cache; public interface to private implementation defined in
 // URLRequestContextConfig class.
 typedef NS_ENUM(NSInteger, CRNHttpCacheType) {
   // Disabled HTTP cache.  Some data may still be temporarily stored in memory.
   CRNHttpCacheTypeDisabled,
   // Enable on-disk HTTP cache, including HTTP data.
   CRNHttpCacheTypeDisk,
   // Enable in-memory cache, including HTTP data.
   CRNHttpCacheTypeMemory,
 };
 
 /// Cronet error domain name.
-NSString* const CRNCronetErrorDomain = @"CRNCronetErrorDomain";
+FOUNDATION_EXPORT GRPC_SUPPORT_EXPORT NSString* const CRNCronetErrorDomain;
 
 /// Enum of Cronet NSError codes.
 NS_ENUM(NSInteger){
     CRNErrorInvalidArgument = 1001,
 };
 
 /// The corresponding value is a String object that contains the name of
 /// an invalid argument inside the NSError userInfo dictionary.
-NSString* const CRNInvalidArgumentKey = @"CRNInvalidArgumentKey";
+FOUNDATION_EXPORT GRPC_SUPPORT_EXPORT NSString* const CRNInvalidArgumentKey;
 
 // A block, that takes a request, and returns YES if the request should
 // be handled.
 typedef BOOL (^RequestFilterBlock)(NSURLRequest* request);
 
 // Interface for installing Cronet.
 // TODO(gcasto): Should this macro be separate from the one defined in
 // bidirectional_stream_c.h?
 GRPC_SUPPORT_EXPORT
 @interface Cronet : NSObject
@@ skipping 143 matching lines @@
 
 // Sets Host Resolver Rules for testing.
 // This method must be called after |start| has been called.
 + (void)setHostResolverRulesForTesting:(NSString*)hostResolverRulesForTesting;
 
 // Enables TestCertVerifier which accepts all certificates for testing.
 // This method only has any effect before |start| is called.
 + (void)enableTestCertVerifierForTesting;
 
 @end