cppgc: Add TraceStrongly to Visitor

Align the library with the current blink implementation.
TraceStrongly takes a WeakMember and strongifies it so that the
referenced objects is retained.
This is used in blink during tracing of some weak collections.

Bug: chromium:1056170
Change-Id: I306f84fc37a856d309bccc7f544750abb2bdc7c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2479003
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70561}

include/cppgc/visitor.h

@@ -148,6 +148,18 @@ class V8_EXPORT Visitor {
     VisitEphemeron(key, value_desc);
   }
 
+  /**
+   * Trace method that strongifies a WeakMember.
+   *
+   * \param weak_member WeakMember reference retaining an object.
+   */
+  template <typename T>
+  void TraceStrongly(const WeakMember<T>& weak_member) {
+    const T* value = weak_member.GetRawAtomic();
+    CPPGC_DCHECK(value != kSentinelPointer);
+    Trace(value);
+  }
+
   /**
    * Registers a weak callback that is invoked during garbage collection.
    *

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

@@ -306,5 +306,18 @@ TEST_F(MarkingVisitorTest, MarkPersistentMixinInConstruction) {
   EXPECT_FALSE(header.IsMarked());
 }
 
+TEST_F(MarkingVisitorTest, StrongTracingMarksWeakMember) {
+  WeakMember<GCed> object(MakeGarbageCollected<GCed>(GetAllocationHandle()));
+  HeapObjectHeader& header = HeapObjectHeader::FromPayload(object);
+
+  TestMarkingVisitor visitor(GetMarker());
+
+  EXPECT_FALSE(header.IsMarked());
+
+  visitor.TraceStrongly(object);
+
+  EXPECT_TRUE(header.IsMarked());
+}
+
 }  // namespace internal
 }  // namespace cppgc