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

components/cronet/ios/Cronet.h

Index: components/cronet/ios/Cronet.h
diff --git a/components/cronet/ios/Cronet.h b/components/cronet/ios/Cronet.h
index d30e27b25c257433ed461fa3951c54f8a6ed748d..cd8483bcff8b4e2783025e83901a7154ac261403 100644
--- a/components/cronet/ios/Cronet.h
+++ b/components/cronet/ios/Cronet.h
@@ -18,7 +18,7 @@ typedef NS_ENUM(NSInteger, CRNHttpCacheType) {
 };
 
 /// Cronet error domain name.
-NSString* const CRNCronetErrorDomain = @"CRNCronetErrorDomain";
+FOUNDATION_EXPORT GRPC_SUPPORT_EXPORT NSString* const CRNCronetErrorDomain;
 
 /// Enum of Cronet NSError codes.
 NS_ENUM(NSInteger){
@@ -27,7 +27,7 @@ NS_ENUM(NSInteger){
 
 /// 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.