Add MockTransformableVideoFrame to api/test/.

Define MockTransformableVideoFrame in api/test to be use in Blink
tests.

Bug: chromium:1069295
Change-Id: I960d8228eddbdf8263b731814a00fd02d0a6c54e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175136
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Marina Ciocea <marinaciocea@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31272}

api/test/compile_all_headers.cc

diff --git a/api/test/compile_all_headers.cc b/api/test/compile_all_headers.cc
index 47c5c6e..d9cc82c 100644
--- a/api/test/compile_all_headers.cc
+++ b/api/test/compile_all_headers.cc
@@ -37,6 +37,7 @@
 #include "api/test/mock_peerconnectioninterface.h"
 #include "api/test/mock_rtpreceiver.h"
 #include "api/test/mock_rtpsender.h"
+#include "api/test/mock_transformable_video_frame.h"
 #include "api/test/mock_video_bitrate_allocator.h"
 #include "api/test/mock_video_bitrate_allocator_factory.h"
 #include "api/test/mock_video_decoder.h"