Avoid crash in ~MultiSourceMemoryPressureMonitor().

Bug: 1008161
Change-Id: Ib8daa0315d3c22c810848d4f696961e7ba005d82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824127Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: Jenny Wong <jyw@google.com>
Cr-Commit-Position: refs/heads/master@{#700311}

base/util/memory_pressure/multi_source_memory_pressure_monitor.cc

@@ -25,7 +25,14 @@ MultiSourceMemoryPressureMonitor::MultiSourceMemoryPressureMonitor()
   StartMetricsTimer();
 }
 
-MultiSourceMemoryPressureMonitor::~MultiSourceMemoryPressureMonitor() = default;
+MultiSourceMemoryPressureMonitor::~MultiSourceMemoryPressureMonitor() {
+  // Destroy system evaluator early while the remaining members of this class
+  // still exist. MultiSourceMemoryPressureMonitor implements
+  // MemoryPressureVoteAggregator::Delegate, and
+  // delegate_->OnMemoryPressureLevelChanged() gets indirectly called during
+  // ~SystemMemoryPressureEvaluator().
+  system_evaluator_.reset();
+}
 
 void MultiSourceMemoryPressureMonitor::StartMetricsTimer() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);