Restore static_assert for MockSessionDescriptionInterface

I mistakenly removed the static_assert in a previous CL:
https://webrtc-review.googlesource.com/c/src/+/407440

Bug: webrtc:442220720
Change-Id: Ied058a47dabc62c65baf513df437f98581743238
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/409381
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45655}
diff --git a/api/test/mock_session_description_interface.h b/api/test/mock_session_description_interface.h
index aea86db..d112426 100644
--- a/api/test/mock_session_description_interface.h
+++ b/api/test/mock_session_description_interface.h
@@ -27,6 +27,7 @@
     "interface.")]] MockSessionDescriptionInterface
     : public SessionDescriptionInterface {
  public:
+  MockSessionDescriptionInterface() = default;
   MOCK_METHOD(std::unique_ptr<SessionDescriptionInterface>,
               Clone,
               (),
@@ -47,6 +48,11 @@
   MOCK_METHOD(bool, ToString, (std::string*), (const, override));
 };
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+static_assert(!std::is_abstract_v<MockSessionDescriptionInterface>);
+#pragma clang diagnostic pop
+
 }  // namespace webrtc
 
 #endif  // API_TEST_MOCK_SESSION_DESCRIPTION_INTERFACE_H_