Fix gRPC-C++ Cronet subspec

templates/gRPC-C++.podspec.template

@@ -199,13 +199,19 @@
       ss.source_files = ${ruby_multiline_list(grpcpp_proto_files(filegroups), 22)}
     end
 
-    s.subspec 'Cronet' do |ss|
+    s.subspec 'Cronet-Interface' do |ss|
+      ss.header_mappings_dir = 'include/grpcpp'
+      ss.public_header_files = "include/grpcpp/security/cronet_credentials.h",
+                               "include/grpcpp/security/cronet_credentials_impl.h"
+      ss.source_files = "include/grpcpp/security/cronet_credentials.h",
+                        "include/grpcpp/security/cronet_credentials_impl.h"
+    end
+
+    s.subspec 'Cronet-Implementation' do |ss|
       ss.header_mappings_dir = 'include/grpcpp'
       ss.dependency 'gRPC-Core/Cronet-Implementation', version
       ss.dependency "#{s.name}/Implementation", version
 
-      ss.public_header_files = "include/grpcpp/security/cronet_credentials.h",
-                               "include/grpcpp/security/cronet_credentials_impl.h"
       ss.source_files = "src/cpp/client/cronet_credentials.cc"
     end