Screen content simulcast layers should not be downscaled.

Fix config so, size isn't downscaled, add unit test coverage.

BUG=webrtc:7171, webrtc:4172

Review-Url: https://codereview.webrtc.org/2692343007
Cr-Commit-Position: refs/heads/master@{#16648}
diff --git a/webrtc/media/engine/webrtcvideoengine2_unittest.cc b/webrtc/media/engine/webrtcvideoengine2_unittest.cc
index f60af34..a65265f 100644
--- a/webrtc/media/engine/webrtcvideoengine2_unittest.cc
+++ b/webrtc/media/engine/webrtcvideoengine2_unittest.cc
@@ -4124,6 +4124,13 @@
       expected_streams = GetSimulcastConfig(
           num_configured_streams, capture_width, capture_height, 0,
           kDefaultQpMax, kDefaultVideoMaxFramerate, screenshare);
+      if (screenshare) {
+        for (const webrtc::VideoStream& stream : expected_streams) {
+          // Never scale screen content.
+          EXPECT_EQ(stream.width, capture_width);
+          EXPECT_EQ(stream.height, capture_height);
+        }
+      }
     } else {
       webrtc::VideoStream stream;
       stream.width = capture_width;