Mark MockSessionDescriptionInterface as deprecated.

This mock class isn't used in WebRTC and it's preferred to use the
SessionDescriptionInterface class directly instead.

Bug: webrtc:442220720
Change-Id: I5b9fc715469b4ad83e407d5a86949bca110cb926
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/407440
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45576}
diff --git a/api/test/mock_session_description_interface.h b/api/test/mock_session_description_interface.h
index e2ff56c..aea86db 100644
--- a/api/test/mock_session_description_interface.h
+++ b/api/test/mock_session_description_interface.h
@@ -21,7 +21,11 @@
 
 namespace webrtc {
 
-class MockSessionDescriptionInterface : public SessionDescriptionInterface {
+class [[deprecated(
+    "Use the SessionDescriptionInterface directly instead, including the "
+    "static Create() methods within the "
+    "interface.")]] MockSessionDescriptionInterface
+    : public SessionDescriptionInterface {
  public:
   MOCK_METHOD(std::unique_ptr<SessionDescriptionInterface>,
               Clone,
@@ -43,8 +47,6 @@
   MOCK_METHOD(bool, ToString, (std::string*), (const, override));
 };
 
-static_assert(!std::is_abstract_v<MockSessionDescriptionInterface>);
-
 }  // namespace webrtc
 
 #endif  // API_TEST_MOCK_SESSION_DESCRIPTION_INTERFACE_H_