Add a new frame generator that cycles through randomly generated slides.

Like YuvFileGenerator, this also updates the display with a new slide on every Nth frame, but it generates the slides itself instead of reading them from files.

BUG=webrtc:8138

Review-Url: https://codereview.webrtc.org/3003193002
Cr-Commit-Position: refs/heads/master@{#19585}
diff --git a/webrtc/test/frame_generator_capturer.cc b/webrtc/test/frame_generator_capturer.cc
index 4f1937b..095a2043 100644
--- a/webrtc/test/frame_generator_capturer.cc
+++ b/webrtc/test/frame_generator_capturer.cc
@@ -114,6 +114,22 @@
   return capturer.release();
 }
 
+FrameGeneratorCapturer* FrameGeneratorCapturer::CreateSlideGenerator(
+    int width,
+    int height,
+    int frame_repeat_count,
+    int target_fps,
+    Clock* clock) {
+  std::unique_ptr<FrameGeneratorCapturer> capturer(new FrameGeneratorCapturer(
+      clock, FrameGenerator::CreateSlideGenerator(width, height,
+                                                  frame_repeat_count),
+      target_fps));
+  if (!capturer->Init())
+    return nullptr;
+
+  return capturer.release();
+}
+
 FrameGeneratorCapturer::FrameGeneratorCapturer(
     Clock* clock,
     std::unique_ptr<FrameGenerator> frame_generator,