Add static factory method from FrameGenerator for FrameGeneratorCapturer.

Add static factory method from FrameGenerator for FrameGeneratorCapturer
to be able to intercept generated frames in PC e2e test framework to
dump input video stream into file, if it was generated.

Bug: webrtc:10138
Change-Id: Iabecfaaef804111e0b19756cd676c1749757d9c6
Reviewed-on: https://webrtc-review.googlesource.com/c/119947
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26424}

test/frame_generator_capturer.cc

@@ -78,6 +78,18 @@ FrameGeneratorCapturer* FrameGeneratorCapturer::CreateSlideGenerator(
   return capturer.release();
 }
 
+FrameGeneratorCapturer* FrameGeneratorCapturer::Create(
+    std::unique_ptr<FrameGenerator> frame_generator,
+    int target_fps,
+    Clock* clock) {
+  auto capturer = absl::make_unique<FrameGeneratorCapturer>(
+      clock, std::move(frame_generator), target_fps);
+  if (!capturer->Init())
+    return nullptr;
+
+  return capturer.release();
+}
+
 FrameGeneratorCapturer::FrameGeneratorCapturer(
     Clock* clock,
     std::unique_ptr<FrameGenerator> frame_generator,

test/frame_generator_capturer.h

@@ -59,6 +59,11 @@ class FrameGeneratorCapturer : public TestVideoCapturer {
                                                       int frame_repeat_count,
                                                       int target_fps,
                                                       Clock* clock);
+  static FrameGeneratorCapturer* Create(
+      std::unique_ptr<FrameGenerator> frame_generator,
+      int target_fps,
+      Clock* clock);
+
   virtual ~FrameGeneratorCapturer();
 
   void Start();