cppgc: Initialize MarkingVisitor's marked bytes as zero

Bug: chromium:1056170
Change-Id: I1e168f967acf0e4d6094106c0693e1a10f409f49
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2231357
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68201}

src/heap/cppgc/marking-visitor.h

@@ -58,7 +58,7 @@ class MarkingVisitor : public VisitorBase, public StackVisitor {
   Marker::NotFullyConstructedWorklist::View not_fully_constructed_worklist_;
   Marker::WeakCallbackWorklist::View weak_callback_worklist_;
 
-  size_t marked_bytes_;
+  size_t marked_bytes_ = 0;
 };
 
 class V8_EXPORT_PRIVATE MutatorThreadMarkingVisitor : public MarkingVisitor {

test/unittests/heap/cppgc/marking-visitor-unittest.cc

@@ -46,6 +46,11 @@ class GCedWithMixin : public GarbageCollected<GCedWithMixin>, public Mixin {
 
 }  // namespace
 
+TEST_F(MarkingVisitorTest, MarkedBytesAreInitiallyZero) {
+  MutatorThreadMarkingVisitor visitor(GetMarker());
+  EXPECT_EQ(0u, visitor.marked_bytes());
+}
+
 // Strong refernces are marked.
 
 TEST_F(MarkingVisitorTest, MarkMember) {