Make frame transformer MimeType pure virtual again

after both audio and video have been implemented.

BUG=webrtc:15579

Change-Id: Ib52e8f67292259cbf7497a884672de72f3003282
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/326162
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tony Herre <herre@google.com>
Cr-Commit-Position: refs/heads/main@{#41114}

api/frame_transformer_interface.h

diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index 535659d..94f8dab 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -13,7 +13,6 @@
 
 #include <memory>
 #include <string>
-#include <vector>
 
 #include "api/ref_count.h"
 #include "api/scoped_refptr.h"
@@ -54,11 +53,7 @@
   // sender frames to allow received frames to be directly re-transmitted on
   // other PeerConnectionss.
   virtual Direction GetDirection() const { return Direction::kUnknown; }
-  virtual std::string GetMimeType() const {
-    // TODO(bugs.webrtc.org/15579): Change this to pure virtual after it
-    // is implemented everywhere.
-    return "unknown/unknown";
-  }
+  virtual std::string GetMimeType() const = 0;
 };
 
 class TransformableVideoFrameInterface : public TransformableFrameInterface {

api/test/mock_transformable_audio_frame.h

diff --git a/api/test/mock_transformable_audio_frame.h b/api/test/mock_transformable_audio_frame.h
index be70300..584c77f 100644
--- a/api/test/mock_transformable_audio_frame.h
+++ b/api/test/mock_transformable_audio_frame.h
@@ -11,6 +11,8 @@
 #ifndef API_TEST_MOCK_TRANSFORMABLE_AUDIO_FRAME_H_
 #define API_TEST_MOCK_TRANSFORMABLE_AUDIO_FRAME_H_
 
+#include <string>
+
 #include "api/frame_transformer_interface.h"
 #include "test/gmock.h"
 
@@ -24,6 +26,7 @@
   MOCK_METHOD(uint8_t, GetPayloadType, (), (const, override));
   MOCK_METHOD(uint32_t, GetSsrc, (), (const, override));
   MOCK_METHOD(uint32_t, GetTimestamp, (), (const, override));
+  MOCK_METHOD(std::string, GetMimeType, (), (const, override));
   MOCK_METHOD(rtc::ArrayView<const uint32_t>,
               GetContributingSources,
               (),