Add trace events that record the memory pressure sessions

Change-Id: Id32ab0a774dc45ca4459e2b709ff82d54389b3c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124949
Auto-Submit: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754496}

base/util/memory_pressure/memory_pressure_voter.cc

@@ -7,6 +7,7 @@
 #include <numeric>
 
 #include "base/stl_util.h"
+#include "base/trace_event/trace_event.h"
 
 namespace util {
 
@@ -83,7 +84,34 @@ void MemoryPressureVoteAggregator::OnVote(
   if (new_vote)
     votes_[new_vote.value()]++;
   auto old_pressure_level = current_pressure_level_;
+
+  // If the pressure level is not None then an asynchronous event will have been
+  // started below, it needs to be ended.
+  // Note that we record this event every time we receive a new vote to ensure
+  // that the begin event doesn't get dropped during long pressure sessions.
+  if (old_pressure_level ==
+      MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_CRITICAL) {
+    TRACE_EVENT_NESTABLE_ASYNC_END0("base", "MemoryPressure::CriticalPressure",
+                                    this);
+  } else if (old_pressure_level ==
+             MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_MODERATE) {
+    TRACE_EVENT_NESTABLE_ASYNC_END0("base", "MemoryPressure::ModeratePressure",
+                                    this);
+  }
+
   current_pressure_level_ = EvaluateVotes();
+
+  // Start an asynchronous tracing event to record this pressure session.
+  if (current_pressure_level_ ==
+      MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_CRITICAL) {
+    TRACE_EVENT_NESTABLE_ASYNC_BEGIN0("base",
+                                      "MemoryPressure::CriticalPressure", this);
+  } else if (current_pressure_level_ ==
+             MemoryPressureLevel::MEMORY_PRESSURE_LEVEL_MODERATE) {
+    TRACE_EVENT_NESTABLE_ASYNC_BEGIN0("base",
+                                      "MemoryPressure::ModeratePressure", this);
+  }
+
   if (old_pressure_level != current_pressure_level_)
     delegate_->OnMemoryPressureLevelChanged(current_pressure_level_);
 }