Update max TL0 frame interval for screensharing.

The previous limit leaved no margin for RTT.

BUG=webrtc:4172

Review-Url: https://codereview.webrtc.org/2911243002
Cr-Commit-Position: refs/heads/master@{#18333}
diff --git a/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc b/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
index ab1fea6..a30679a 100644
--- a/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
+++ b/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
@@ -35,7 +35,7 @@
 
 // Always emit a frame with certain interval, even if bitrate targets have
 // been exceeded. This prevents needless keyframe requests.
-const int ScreenshareLayers::kMaxFrameIntervalMs = 3000;
+const int ScreenshareLayers::kMaxFrameIntervalMs = 2750;
 
 webrtc::TemporalLayers* ScreenshareTemporalLayersFactory::Create(
     int simulcast_id,