cppgc: Fix allocated_memory_size()

Bug: chromium:1056170
Change-Id: I4b43b78be23b8220b89f1ee5177f6b9b3c18146d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2764748
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73452}

src/heap/cppgc/stats-collector.cc

@@ -224,7 +224,7 @@ void StatsCollector::NotifySweepingCompleted() {
 }
 
 size_t StatsCollector::allocated_memory_size() const {
-  return memory_allocated_bytes_;
+  return memory_allocated_bytes_ - memory_freed_bytes_since_end_of_marking_;
 }
 
 size_t StatsCollector::allocated_object_size() const {

test/unittests/heap/cppgc/stats-collector-unittest.cc

@@ -227,5 +227,13 @@ TEST_F(StatsCollectorTest, ObserverTriggersGC) {
   stats.UnregisterObserver(&mock_observer);
 }
 
+TEST_F(StatsCollectorTest, AllocatedMemorySize) {
+  EXPECT_EQ(0u, stats.allocated_memory_size());
+  stats.NotifyAllocatedMemory(1024);
+  EXPECT_EQ(1024u, stats.allocated_memory_size());
+  stats.NotifyFreedMemory(1024);
+  EXPECT_EQ(0u, stats.allocated_memory_size());
+}
+
 }  // namespace internal
 }  // namespace cppgc