Add a Start function to MultiSourceMemoryPressureMonitor

Change-Id: I8cfd7c201e847596d54b750aa24bb7606b2d8498
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836019Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702610}

base/util/memory_pressure/multi_source_memory_pressure_monitor.cc

@@ -17,12 +17,6 @@ MultiSourceMemoryPressureMonitor::MultiSourceMemoryPressureMonitor()
       dispatch_callback_(base::BindRepeating(
           &base::MemoryPressureListener::NotifyMemoryPressure)),
       aggregator_(this) {
-  // This can't be in the parameter list because |sequence_checker_| wouldn't be
-  // available, which would be needed by the |system_evaluator_|'s constructor's
-  // call to CreateVoter().
-  system_evaluator_ =
-      SystemMemoryPressureEvaluator::CreateDefaultSystemEvaluator(this);
-  StartMetricsTimer();
 }
 
 MultiSourceMemoryPressureMonitor::~MultiSourceMemoryPressureMonitor() {
@@ -34,6 +28,12 @@ MultiSourceMemoryPressureMonitor::~MultiSourceMemoryPressureMonitor() {
   system_evaluator_.reset();
 }
 
+void MultiSourceMemoryPressureMonitor::Start() {
+  system_evaluator_ =
+      SystemMemoryPressureEvaluator::CreateDefaultSystemEvaluator(this);
+  StartMetricsTimer();
+}
+
 void MultiSourceMemoryPressureMonitor::StartMetricsTimer() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   // Unretained is safe here since this task is running on a timer owned by this

base/util/memory_pressure/multi_source_memory_pressure_monitor.h

@@ -30,6 +30,9 @@ class MultiSourceMemoryPressureMonitor
   MultiSourceMemoryPressureMonitor();
   ~MultiSourceMemoryPressureMonitor() override;
 
+  // Start monitoring memory pressure using the platform-specific voter.
+  void Start();
+
   // MemoryPressureMonitor implementation.
   MemoryPressureLevel GetCurrentPressureLevel() const override;
   void SetDispatchCallback(const DispatchCallback& callback) override;

base/util/memory_pressure/multi_source_memory_pressure_monitor_unittest.cc

@@ -14,6 +14,7 @@ namespace util {
 TEST(MultiSourceMemoryPressureMonitorTest, RunDispatchCallback) {
   base::test::TaskEnvironment task_environment;
   MultiSourceMemoryPressureMonitor monitor;
+  monitor.Start();
   auto* aggregator = monitor.aggregator_for_testing();
 
   bool callback_called = false;

content/browser/browser_main_loop.cc

@@ -21,6 +21,7 @@
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/memory/memory_pressure_monitor.h"
+#include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/message_loop/message_loop_current.h"
 #include "base/metrics/field_trial.h"
@@ -366,14 +367,20 @@ std::unique_ptr<base::MemoryPressureMonitor> CreateMemoryPressureMonitor(
   if (command_line.HasSwitch(switches::kBrowserTest))
     return nullptr;
 
+  std::unique_ptr<util::MultiSourceMemoryPressureMonitor> monitor;
+
 #if defined(OS_CHROMEOS)
   if (chromeos::switches::MemoryPressureHandlingEnabled())
-    return std::make_unique<util::MultiSourceMemoryPressureMonitor>();
+    monitor = std::make_unique<util::MultiSourceMemoryPressureMonitor>();
 #elif defined(OS_MACOSX) || defined(OS_WIN)
-  return std::make_unique<util::MultiSourceMemoryPressureMonitor>();
+  monitor = std::make_unique<util::MultiSourceMemoryPressureMonitor>();
 #endif
   // No memory monitor on other platforms...
-  return nullptr;
+
+  if (monitor)
+    monitor->Start();
+
+  return monitor;
 }
 
 #if defined(OS_CHROMEOS)