Wire up RTCOutboundRtpStreamStats.totalEncodeTime.

This is a follow-up to
https://webrtc-review.googlesource.com/c/src/+/130517 that calculated
this metric.

This CL is purely plumbing, exposing
VideoSendStream::total_encode_time_ms in standard getStats() as
RTCOutboundRtpStreamStats.totalEncodeTime (in seconds):
https://w3c.github.io/webrtc-stats/#dom-rtcoutboundrtpstreamstats-totalencodetime

Bug: webrtc:10448
Change-Id: I715f1ef937e441169dee55b5e8d4fbf98811c5f3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/131940
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27501}
diff --git a/pc/rtc_stats_integrationtest.cc b/pc/rtc_stats_integrationtest.cc
index 5fd353c..13eb213 100644
--- a/pc/rtc_stats_integrationtest.cc
+++ b/pc/rtc_stats_integrationtest.cc
@@ -762,8 +762,11 @@
     if (outbound_stream.media_type.is_defined() &&
         *outbound_stream.media_type == "video") {
       verifier.TestMemberIsDefined(outbound_stream.frames_encoded);
+      verifier.TestMemberIsNonNegative<double>(
+          outbound_stream.total_encode_time);
     } else {
       verifier.TestMemberIsUndefined(outbound_stream.frames_encoded);
+      verifier.TestMemberIsUndefined(outbound_stream.total_encode_time);
     }
     return verifier.ExpectAllMembersSuccessfullyTested();
   }