Add GetDirection method to MockTransformableVideoFrame

Allow mocking of GetDirection in tests.
Required for Chromium adoption of this mock:
https://chromium-review.googlesource.com/c/chromium/src/+/4236916

Bug: chromium:1414370
Change-Id: I2e7443a1bf24966cfcfaeadf47c5b29375e84f99
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/293745
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Tony Herre <herre@google.com>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39329}

api/test/mock_transformable_video_frame.h

@@ -36,6 +36,10 @@ class MockTransformableVideoFrame
               (const webrtc::VideoFrameMetadata&),
               (override));
   MOCK_METHOD(uint8_t, GetPayloadType, (), (const, override));
+  MOCK_METHOD(TransformableFrameInterface::Direction,
+              GetDirection,
+              (),
+              (const, override));
 };
 
 static_assert(!std::is_abstract_v<MockTransformableVideoFrame>, "");