Add mock of GetCaptureTimeIdentifier to MockTransformableVideoFrame

Bug: webrtc:14878
Change-Id: I2dffad0932aee4d2ba37c8d57a3c28330e3cc294
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/316880
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Auto-Submit: Tony Herre <herre@google.com>
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40585}

api/test/mock_transformable_video_frame.h

@@ -37,6 +37,10 @@ class MockTransformableVideoFrame
               (),
               (const, override));
   MOCK_METHOD(VideoFrameMetadata, Metadata, (), (const, override));
+  MOCK_METHOD(absl::optional<Timestamp>,
+              GetCaptureTimeIdentifier,
+              (),
+              (const, override));
 };
 
 static_assert(!std::is_abstract_v<MockTransformableVideoFrame>, "");