Add metrics regarding concurrent audible tabs in Chromium.

content/browser/media/audible_metrics.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "content/browser/media/audible_metrics.h"
+
+#include "base/metrics/histogram_macros.h"
+#include "base/metrics/user_metrics.h"
+#include "base/time/default_tick_clock.h"
+
+namespace content {
+
+AudibleMetrics::AudibleMetrics()
+    : max_concurrent_audible_web_contents_in_session_(0),
+      clock_(new base::DefaultTickClock()) {
+}
+
+AudibleMetrics::~AudibleMetrics() {
+}
+
+void AudibleMetrics::UpdateAudibleWebContentsState(
+    const WebContents* web_contents, bool audible) {
+  bool found =
+      audible_web_contents_.find(web_contents) != audible_web_contents_.end();
+  if (found == audible)
+    return;
+
+  if (audible)
+    AddAudibleWebContents(web_contents);
+  else
+    RemoveAudibleWebContents(web_contents);
+}
+
+void AudibleMetrics::SetClockForTest(scoped_ptr<base::TickClock> test_clock) {
+  clock_ = std::move(test_clock);
+}
+
+void AudibleMetrics::AddAudibleWebContents(const WebContents* web_contents) {
+  base::RecordAction(base::UserMetricsAction("Media.Audible.AddTab"));
+
+  UMA_HISTOGRAM_CUSTOM_COUNTS(
+      "Media.Audible.ConcurrentTabsWhenStarting", audible_web_contents_.size(),
+      0, 10, 11);

[Mark P, 2016/01/20 00:19:16]

the min bucket should be 1.
(We always have an "underflow" bucket that contains 0.)

[mlamouri (slow - plz ping), 2016/01/20 11:31:52]

Done.

+
+  audible_web_contents_.insert(web_contents);
+  if (audible_web_contents_.size() > 1 &&
+      concurrent_web_contents_start_time_.is_null()) {
+    concurrent_web_contents_start_time_ = clock_->NowTicks();
+  }
+
+  if (audible_web_contents_.size() >
+      max_concurrent_audible_web_contents_in_session_) {
+    max_concurrent_audible_web_contents_in_session_ =
+        audible_web_contents_.size();
+
+    UMA_HISTOGRAM_CUSTOM_COUNTS(
+        "Media.Audible.ConcurrentTabsInSession",
+        max_concurrent_audible_web_contents_in_session_,
+        1, 10, 10);

[Mark P, 2016/01/20 00:19:15]

nit: second 10 here might as well be 11 for consistency with the above call.
(in practice, it won't make much difference.)

[mlamouri (slow - plz ping), 2016/01/20 11:31:52]

Done.

+  }
+}
+
+void AudibleMetrics::RemoveAudibleWebContents(const WebContents* web_contents) {
+  base::RecordAction(base::UserMetricsAction("Media.Audible.RemoveTab"));
+
+  audible_web_contents_.erase(web_contents);
+
+  if (audible_web_contents_.size() <= 1 &&
+      !concurrent_web_contents_start_time_.is_null()) {
+    base::TimeDelta concurrent_total_time =
+        clock_->NowTicks() - concurrent_web_contents_start_time_;
+    concurrent_web_contents_start_time_ = base::TimeTicks();
+
+    UMA_HISTOGRAM_LONG_TIMES("Media.Audible.ConcurrentTabsTime",
+                             concurrent_total_time);
+  }
+}
+
+}  // namespace content