Use SubchannelInterface to hide implementation from LB policy API.
diff --git a/gRPC-Core.podspec b/gRPC-Core.podspec
index 8ad00aa..85add8f 100644
--- a/gRPC-Core.podspec
+++ b/gRPC-Core.podspec
@@ -371,6 +371,7 @@
                       'src/core/ext/filters/client_channel/server_address.h',
                       'src/core/ext/filters/client_channel/service_config.h',
                       'src/core/ext/filters/client_channel/subchannel.h',
+                      'src/core/ext/filters/client_channel/subchannel_interface.h',
                       'src/core/ext/filters/client_channel/subchannel_pool_interface.h',
                       'src/core/ext/filters/deadline/deadline_filter.h',
                       'src/core/ext/filters/client_channel/health/health.pb.h',
@@ -1023,6 +1024,7 @@
                               'src/core/ext/filters/client_channel/server_address.h',
                               'src/core/ext/filters/client_channel/service_config.h',
                               'src/core/ext/filters/client_channel/subchannel.h',
+                              'src/core/ext/filters/client_channel/subchannel_interface.h',
                               'src/core/ext/filters/client_channel/subchannel_pool_interface.h',
                               'src/core/ext/filters/deadline/deadline_filter.h',
                               'src/core/ext/filters/client_channel/health/health.pb.h',