Add inter frame delay standard deviation to WebRTC internals page

This CL whitelists the WebRTC statistics total (squared) inter frame delay
and adds two derived statistics, average inter frame delay and the
standard deviation of the inter frame delay.

Bug: webrtc:11108
Change-Id: I1b8e4c38a0093e622341f67590e930102a534d5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929819Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Johannes Kron <kron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718285}

chrome/test/data/webrtc/peerconnection_getstats.js

@@ -119,6 +119,8 @@ let kRTCInboundRtpStreamStats = new RTCStats(kRTCReceivedRtpStreamStats, {
   keyFramesDecoded: 'number',
   qpSum: 'number',
   totalDecodeTime: 'number',
+  totalInterFrameDelay: 'number',
+  totalSquaredInterFrameDelay: 'number',
   lastPacketReceivedTimestamp: 'number',
   averageRtcpInterval: 'number',
   fecPacketsReceived: 'number',

content/browser/resources/media/stats_rates_calculator.js

@@ -318,6 +318,71 @@ class DifferenceCalculator {
   }
 }
 
+// Calculates the standard deviation from a totalSquaredSum, totalSum, and
+// totalCount. If the standard deviation cannot be calculated, such as the
+// metric is missing in the current or previous report, undefined is returned.
+class StandardDeviationCalculator {
+  constructor(totalSquaredSumMetric, totalSumMetric, totalCount, label) {
+    this.totalSquaredSumMetric = totalSquaredSumMetric;
+    this.totalSumMetric = totalSumMetric;
+    this.totalCount = totalCount;
+    this.label = label;
+  }
+
+  getCalculatedMetricName() {
+    return '[' + this.label + 'StDev_in_ms]';
+  }
+
+  calculate(id, previousReport, currentReport) {
+    return StandardDeviationCalculator.calculateStandardDeviation(
+        id, previousReport, currentReport, this.totalSquaredSumMetric,
+        this.totalSumMetric, this.totalCount);
+  }
+
+  static calculateStandardDeviation(
+      id, previousReport, currentReport, totalSquaredSumMetric, totalSumMetric,
+      totalCount) {
+    if (!previousReport || !currentReport) {
+      return undefined;
+    }
+    const previousStats = previousReport.get(id);
+    const currentStats = currentReport.get(id);
+    if (!previousStats || !currentStats) {
+      return undefined;
+    }
+    const deltaCount =
+        Number(currentStats[totalCount]) - Number(previousStats[totalCount]);
+    if (deltaCount <= 0) {
+      return undefined;
+    }
+    // Try to convert whatever the values are to numbers. This gets around the
+    // fact that some types that are not supported by base::Value (e.g. uint32,
+    // int64, uint64 and double) are passed as strings.
+    const previousSquaredSumValue =
+        Number(previousStats[totalSquaredSumMetric]);
+    const currentSquaredSumValue = Number(currentStats[totalSquaredSumMetric]);
+    if (typeof previousSquaredSumValue != 'number' ||
+        typeof currentSquaredSumValue != 'number') {
+      return undefined;
+    }
+    const previousSumValue = Number(previousStats[totalSumMetric]);
+    const currentSumValue = Number(currentStats[totalSumMetric]);
+    if (typeof previousSumValue != 'number' ||
+        typeof currentSumValue != 'number') {
+      return undefined;
+    }
+
+    const deltaSquaredSum = currentSquaredSumValue - previousSquaredSumValue;
+    const deltaSum = currentSumValue - previousSumValue;
+    const variance =
+        (deltaSquaredSum - Math.pow(deltaSum, 2) / deltaCount) / deltaCount;
+    if (variance < 0) {
+      return undefined;
+    }
+    return 1000 * Math.sqrt(variance);
+  }
+}
+
 // Keeps track of previous and current stats report and calculates all
 // calculated metrics.
 class StatsRatesCalculator {
@@ -393,6 +458,12 @@ class StatsRatesCalculator {
           totalDecodeTime: new RateCalculator(
               'totalDecodeTime', 'framesDecoded',
               CalculatorModifier.kMillisecondsFromSeconds),
+          totalInterFrameDelay: new RateCalculator(
+              'totalInterFrameDelay', 'framesDecoded',
+              CalculatorModifier.kMillisecondsFromSeconds),
+          totalSquaredInterFrameDelay: new StandardDeviationCalculator(
+              'totalSquaredInterFrameDelay', 'totalInterFrameDelay',
+              'framesDecoded', 'interFrameDelay'),
           qpSum: new RateCalculator('qpSum', 'framesDecoded'),
           codecId: new CodecCalculator(),
         },