Add multithreaded fake encoder and corresponding FlexFEC VideoSendStreamTest.

webrtc/test/fake_encoder.h

Index: webrtc/test/fake_encoder.h
diff --git a/webrtc/test/fake_encoder.h b/webrtc/test/fake_encoder.h
index 57b39a4404ed857b3e4c4cc7323e9e1f2fb0034e..e978d3ac313997d1b7c7ada222268ae9798d05e1 100644
--- a/webrtc/test/fake_encoder.h
+++ b/webrtc/test/fake_encoder.h
@@ -14,6 +14,7 @@
 #include <vector>
 
 #include "webrtc/base/criticalsection.h"
+#include "webrtc/base/task_queue.h"
 #include "webrtc/common_types.h"
 #include "webrtc/system_wrappers/include/clock.h"
 #include "webrtc/video_encoder.h"
@@ -86,6 +87,31 @@ class DelayedEncoder : public test::FakeEncoder {
   rtc::CriticalSection lock_;
   int delay_ms_ GUARDED_BY(&lock_);
 };
+
+// This class implements a multi-threaded fake encoder by posting
+// FakeEncoder::Encode(.) tasks to |queue1_| and |queue2_|, in an
+// alternating fashion.
+class MultiThreadedFakeEncoder : public test::FakeEncoder {
+ public:
+  MultiThreadedFakeEncoder(Clock* clock);
+  virtual ~MultiThreadedFakeEncoder() override;
+
+  int32_t Encode(const VideoFrame& input_image,
+                 const CodecSpecificInfo* codec_specific_info,
+                 const std::vector<FrameType>* frame_types) override;
+
+  int32_t EncodeCallback(const VideoFrame& input_image,
+                         const CodecSpecificInfo* codec_specific_info,
+                         const std::vector<FrameType>* frame_types);
+
+ protected:
+  class EncodeTask;
+
+  int current_queue_;
+  rtc::TaskQueue queue1_;
+  rtc::TaskQueue queue2_;
+};
+
 }  // namespace test
 }  // namespace webrtc