Calculate average packet send-side delay in chrome://webrtc-internals.

When https://webrtc-review.googlesource.com/c/src/+/137044/ lands, this
will show the packet average send-side delay over the last second as a
graph in chrome://webrtc-internals. Verified manually.

Bug: webrtc:10506
Change-Id: Icade78b31ed37509e8769b6b9b9c4b89447c619a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615144Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660852}

content/browser/resources/media/stats_rates_calculator.js

@@ -189,6 +189,14 @@ class StatsRatesCalculator {
         names: [
           ['bytesSent', 'timestamp'],
           ['packetsSent', 'timestamp'],
+          [
+            'totalPacketSendDelay',
+            'packetsSent',
+            '[totalPacketSendDelay/packetsSent_in_ms]',
+            (value) => {
+              return value * 1000;  // s -> ms
+            },
+          ],
           ['framesEncoded', 'timestamp'],
           ['qpSum', 'framesEncoded'],
         ],