Use simulcast for screenshare only in conference mode

Also, don't crash if InitEncode fails for vp8.

BUG=chromium:705505

Review-Url: https://codereview.webrtc.org/2779163002
Cr-Commit-Position: refs/heads/master@{#17452}

webrtc/media/engine/webrtcvideoengine2_unittest.cc

@@ -4360,4 +4360,12 @@ TEST_F(WebRtcVideoChannel2SimulcastTest, SetSendCodecsForSimulcastScreenshare) {
                           true);
 }
 
+TEST_F(WebRtcVideoChannel2SimulcastTest,
+       NoSimulcastScreenshareWithoutConference) {
+  webrtc::test::ScopedFieldTrials override_field_trials_(
+      "WebRTC-SimulcastScreenshare/Enabled/");
+  VerifySimulcastSettings(cricket::VideoCodec("VP8"), 1280, 720, 3, 1, true,
+                          false);
+}
+
 }  // namespace cricket