Add MemoryPressureLevelReporter

This observes memory pressure level changes to report the time spent in
each pressure state. The goal is to use this to get rid of the repeating
timer currently used to report Memory.PressureLevel.

The MemoryPressureMonitor is owned by content::BrowserMainLoop and torn
down in content::BrowserMainLoop::ShutdownThreadsAndCleanUp, that should
let Chrome send the data for the latest memory pressure interval at
shutdown.

Bug: 1195080
Change-Id: I2aa406f3524d7dfc22e10c5f9dbc76c060a7ec6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2860304
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#878435}

diff --git a/base/util/memory_pressure/multi_source_memory_pressure_monitor.cc b/base/util/memory_pressure/multi_source_memory_pressure_monitor.cc
index 32cbc1e..6b725e2 100644
--- a/base/util/memory_pressure/multi_source_memory_pressure_monitor.cc
+++ b/base/util/memory_pressure/multi_source_memory_pressure_monitor.cc

@@ -24,8 +24,8 @@
           base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE),
       dispatch_callback_(base::BindRepeating(
           &base::MemoryPressureListener::NotifyMemoryPressure)),
-      aggregator_(this) {
-}
+      aggregator_(this),
+      level_reporter_(current_pressure_level_) {}
 
 MultiSourceMemoryPressureMonitor::~MultiSourceMemoryPressureMonitor() {
   // Destroy system evaluator early while the remaining members of this class
@@ -86,6 +86,8 @@
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   DCHECK_NE(current_pressure_level_, level);
 
+  level_reporter_.OnMemoryPressureLevelChanged(level);
+
   TRACE_EVENT_INSTANT(
       "base", "MultiSourceMemoryPressureMonitor::OnMemoryPressureLevelChanged",
       [&](perfetto::EventContext ctx) {
@@ -95,52 +97,6 @@
             base::trace_event::MemoryPressureLevelToTraceEnum(level));
       });
 
-  // Records the duration of the latest pressure session, there are 4
-  // transitions of interest:
-  //   - Moderate -> None
-  //   - Moderate -> Critical
-  //   - Critical -> Moderate
-  //   - Critical -> None
-
-  base::TimeTicks now = base::TimeTicks::Now();
-
-  if (current_pressure_level_ !=
-      MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_NONE) {
-    DCHECK(!last_pressure_change_timestamp_.is_null());
-    std::string histogram_name = "Memory.PressureWindowDuration.";
-    switch (current_pressure_level_) {
-      // From:
-      case MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_MODERATE: {
-        // To:
-        if (level == MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_NONE) {
-          histogram_name += "ModerateToNone";
-        } else {  // MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_CRITICAL
-          histogram_name += "ModerateToCritical";
-        }
-        break;
-      }
-      // From:
-      case MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_CRITICAL: {
-        // To:
-        if (level == MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_NONE) {
-          histogram_name += "CriticalToNone";
-        } else {  // MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_MODERATE
-          histogram_name += "CriticalToModerate";
-        }
-        break;
-      }
-      case MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_NONE:
-      default:
-        break;
-    }
-
-    base::UmaHistogramCustomTimes(
-        histogram_name, now - last_pressure_change_timestamp_,
-        base::TimeDelta::FromSeconds(1), base::TimeDelta::FromMinutes(10), 50);
-  }
-
-  last_pressure_change_timestamp_ = now;
-
   current_pressure_level_ = level;
 }