New statistic added to VideoReceiveStream to determine latency to first decode.

This change introduces a new measurement into the VideoReceiveStream::Stats
structure to measure the latency between the first frame being received and
the first frame being decoded in WebRTC. The goal here is to measure the latency
difference when a FrameEncryptor is attached and not attached.

Change-Id: I0f0178aff73b66f25dbc6617098033e226da2958
Bug: webrtc:10105
Reviewed-on: https://webrtc-review.googlesource.com/c/113328
Commit-Queue: Benjamin Wright <benwright@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25956}
diff --git a/api/statstypes.h b/api/statstypes.h
index 857b1af..e653d15 100644
--- a/api/statstypes.h
+++ b/api/statstypes.h
@@ -176,6 +176,7 @@
     kStatsValueNameFingerprintAlgorithm,
     kStatsValueNameFirsReceived,
     kStatsValueNameFirsSent,
+    kStatsValueNameFirstFrameReceivedToDecodedMs,
     kStatsValueNameFrameHeightInput,
     kStatsValueNameFrameHeightReceived,
     kStatsValueNameFrameHeightSent,