Add a Start function to MultiSourceMemoryPressureMonitor

Change-Id: I8cfd7c201e847596d54b750aa24bb7606b2d8498
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836019
Reviewed-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}

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 beb507f..2b9a2c4 100644
--- a/base/util/memory_pressure/multi_source_memory_pressure_monitor.cc
+++ b/base/util/memory_pressure/multi_source_memory_pressure_monitor.cc

@@ -17,12 +17,6 @@
       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 @@
   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