Revert "frame transformer: make GetPayloadType pure virtual again"

This reverts commit 209ac5fd95594ab3834dad3e3dbd14c8196637bc.

Reason for revert: Breaks WebRTC autoroll presubmit:
https://chromium-review.googlesource.com/c/chromium/src/+/3134502
Example failure https://ci.chromium.org/ui/p/chromium/builders/try/mac-rel/775468/overview

../../buildtools/third_party/libc++/trunk/include/__memory/unique_ptr.h:725:32: error: allocating an object of abstract class type 'testing::NiceMock<blink::(anonymous namespace)::MockTransformableVideoFrame>'
    return unique_ptr<_Tp>(new _Tp(_VSTD::forward<_Args>(__args)...));
                               ^
../../third_party/blink/renderer/platform/peerconnection/rtc_encoded_video_stream_transformer_test.cc:69:26: note: in instantiation of function template specialization 'std::make_unique<testing::NiceMock<blink::(anonymous namespace)::MockTransformableVideoFrame>>' requested here
  auto mock_frame = std::make_unique<NiceMock<MockTransformableVideoFrame>>();
                         ^
../../third_party/webrtc/api/frame_transformer_interface.h:36:19: note: unimplemented pure virtual method 'GetPayloadType' in 'NiceMock'
  virtual uint8_t GetPayloadType() const = 0;
                  ^


Original change's description:
> frame transformer: make GetPayloadType pure virtual again
>
> after chrome was updated in
>   https://chromium-review.googlesource.com/c/chromium/src/+/3103323
>
> BUG=webrtc:13077
>
> Change-Id: I7e5ff6aaae81c5dcfbaa41b09ef01bc95bb7251a
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/230143
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
> Cr-Commit-Position: refs/heads/main@{#34877}

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: webrtc:13077
Change-Id: I6b2e4e2804890c857f1f832a6a4faa614ec026c4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/230920
Reviewed-by: Olga Sharonova <olka@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Olga Sharonova <olka@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34891}
diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index 7653bee..ca3a15a 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -33,7 +33,9 @@
   // Copies `data` into the owned frame payload data.
   virtual void SetData(rtc::ArrayView<const uint8_t> data) = 0;
 
-  virtual uint8_t GetPayloadType() const = 0;
+  // TODO(bugs.webrtc.org/13077) make pure virtual again after rolling into
+  // Chrome.
+  virtual uint8_t GetPayloadType() const { return 255; }
   virtual uint32_t GetSsrc() const = 0;
   virtual uint32_t GetTimestamp() const = 0;
 };