Whitelist RTCInboundRtpStreamStats::estimatedPlayoutTimestamp.

This unblocks https://webrtc-review.googlesource.com/c/src/+/155621 from
landing and rolling into chromium.

TBR=asapersson@chromium.org, hta@chromium.org
NOTRY=True

Bug: webrtc:7065
Change-Id: I59f9779c265bd3e1c6f24ae3a377a0d18cf95fbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871430Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707737}

chrome/test/data/webrtc/peerconnection_getstats.js

@@ -132,6 +132,7 @@ let kRTCInboundRtpStreamStats = new RTCStats(kRTCReceivedRtpStreamStats, {
   firCount: 'number',
   pliCount: 'number',
   sliCount: 'number',
+  estimatedPlayoutTimestamp: 'number',
   fractionLost: 'number',  // Obsolete, moved to RTCRemoteInboundRtpStreamStats.
   decoderImplementation: 'string',
 });