Fixing loopback video test by reconfiguring the encoder to correct size.

Same as https://codereview.webrtc.org/2480753002, but with a small fix.

BUG=none

Review-Url: https://codereview.webrtc.org/2578143002
Cr-Commit-Position: refs/heads/master@{#15639}

webrtc/video/video_quality_test.cc

@@ -103,7 +103,11 @@ class VideoStreamFactory
       int width,
       int height,
       const webrtc::VideoEncoderConfig& encoder_config) override {
-    return streams_;
+    // The highest layer must match the incoming resolution.
+    std::vector<webrtc::VideoStream> streams = streams_;
+    streams[streams_.size() - 1].height = height;
+    streams[streams_.size() - 1].width = width;
+    return streams;
   }
 
   std::vector<webrtc::VideoStream> streams_;