Add constructor from test::FrameGeneratorCapturer

Bug: webrtc:10138
Change-Id: I55cac374c1cf07cfeac8c54b3ff0ceb995c95e18
Reviewed-on: https://webrtc-review.googlesource.com/c/115760
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26111}

pc/test/framegeneratorcapturervideotracksource.h

@@ -12,6 +12,7 @@
 #define PC_TEST_FRAMEGENERATORCAPTURERVIDEOTRACKSOURCE_H_
 
 #include <memory>
+#include <utility>
 
 #include "pc/videotracksource.h"
 #include "test/frame_generator_capturer.h"
@@ -46,6 +47,11 @@ class FrameGeneratorCapturerVideoTrackSource : public VideoTrackSource {
         config.num_squares_generated, config.frames_per_second, clock));
   }
 
+  explicit FrameGeneratorCapturerVideoTrackSource(
+      std::unique_ptr<test::FrameGeneratorCapturer> video_capturer)
+      : VideoTrackSource(false /* remote */),
+        video_capturer_(std::move(video_capturer)) {}
+
   ~FrameGeneratorCapturerVideoTrackSource() = default;
 
   void Start() {