Using static frame generator when no scrolling

In screensharing full stack tests, instead of using YuvFileGenerator by default
when no scrolling is used, I always used ScrollingImageFileGenerator.
That possibly slowed down the test a little bit, at least for the slowed
devices, as it unnecessarily copied few MBs per frame.

BUG=chromium:534220

Review URL: https://codereview.webrtc.org/1359783002

Cr-Commit-Position: refs/heads/master@{#10014}
diff --git a/webrtc/video/video_quality_test.cc b/webrtc/video/video_quality_test.cc
index cbd1e60..5837660 100644
--- a/webrtc/video/video_quality_test.cc
+++ b/webrtc/video/video_quality_test.cc
@@ -677,11 +677,17 @@
     RTC_CHECK_LE(params.screenshare.scroll_duration,
                  params.screenshare.slide_change_interval);
 
-    frame_generator_.reset(
-        test::FrameGenerator::CreateScrollingInputFromYuvFiles(
-            clock_, slides, kWidth, kHeight, params.common.width,
-            params.common.height, params.screenshare.scroll_duration * 1000,
-            kPauseDurationMs));
+    if (params.screenshare.scroll_duration) {
+      frame_generator_.reset(
+          test::FrameGenerator::CreateScrollingInputFromYuvFiles(
+              clock_, slides, kWidth, kHeight, params.common.width,
+              params.common.height, params.screenshare.scroll_duration * 1000,
+              kPauseDurationMs));
+    } else {
+      frame_generator_.reset(test::FrameGenerator::CreateFromYuvFile(
+              slides, kWidth, kHeight,
+              params.screenshare.slide_change_interval * params.common.fps));
+    }
   }
 }