Adds UMA metrics for renderer uptime

chrome/browser/metrics/renderer_uptime_tracker.cc

+// Copyright 2017 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 "chrome/browser/metrics/renderer_uptime_tracker.h"
+
+#include "base/metrics/histogram_macros.h"
+
+namespace {
+
+RendererUptimeTracker* g_instance = nullptr;
+
+}  // namespace
+
+// static
+void RendererUptimeTracker::Initialize() {

[Alexei Svitkine (slow), 2017/02/28 16:13:12]

DCHECK(!g_instance);

[keishi, 2017/03/01 08:09:54]

Done.

+  g_instance = new RendererUptimeTracker;
+}
+
+// static
+bool RendererUptimeTracker::IsInitialized() {
+  return g_instance != nullptr;
+}
+
+// static
+RendererUptimeTracker* RendererUptimeTracker::Get() {
+  DCHECK(g_instance);
+  return g_instance;
+}
+
+RendererUptimeTracker::RendererUptimeTracker() {}
+
+RendererUptimeTracker::~RendererUptimeTracker() {}
+
+void RendererUptimeTracker::OnRendererStarted(int pid) {
+  info_map_[pid] = RendererInfo{base::TimeTicks::Now(), 0, 0};
+}
+
+void RendererUptimeTracker::OnRendererTerminated(int pid) {
+  auto it = info_map_.find(pid);
+  if (it != info_map_.end()) {
+    auto uptime = base::TimeTicks::Now() - it->second.launched_at_;
+    UMA_HISTOGRAM_CUSTOM_TIMES("Memory.Experimental.Renderer.Uptime", uptime,
+                               base::TimeDelta::FromHours(1),
+                               base::TimeDelta::FromDays(7), 100);

[Alexei Svitkine (slow), 2017/02/28 16:13:12]

We strongly discourage 100 bucket histograms now. Can you use 50 instead?

[keishi, 2017/03/01 08:09:54]

Done.

+    UMA_HISTOGRAM_COUNTS_10000(
+        "Memory.Experimental.Renderer.LoadsInMainFrameDuringUptime",
+        it->second.num_loads_in_main_frame_);
+    info_map_.erase(it);
+  }
+}
+
+void RendererUptimeTracker::OnLoadInMainFrame(int pid) {
+  auto it = info_map_.find(pid);
+  if (it != info_map_.end())

[Alexei Svitkine (slow), 2017/02/28 16:13:11]

What does it mean if it's not found? Should it be a DCHECK()? Or if it's a valid
case, add a comment explaining.

[keishi, 2017/03/01 08:09:54]

Done. Changed to DCHECK. I was unsure if metrics turn on/off dynamically.

+    it->second.num_loads_in_main_frame_++;
+}