Polish EmbedderHeapTracer and move some checks from blink to v8

Quick one, ptal.

BUG=468240
LOG=no

Review URL: https://codereview.chromium.org/1895753002

Cr-Commit-Position: refs/heads/master@{#35578}

include/v8.h

@@ -5413,16 +5413,16 @@ enum class MemoryPressureLevel { kNone, kModerate, kCritical };
  * trace through its heap and call PersistentBase::RegisterExternalReference on
  * each js object reachable from any of the given wrappers.
  *
- * Before the first call to the TraceWrappableFrom function v8 will call
- * TraceRoots. When the v8 garbage collection is finished, v8 will call
- * ClearTracingMarks.
+ * Before the first call to the TraceWrappersFrom function v8 will call
+ * TracePrologue. When the v8 garbage collection is finished, v8 will call
+ * TraceEpilogue.
  */
 class EmbedderHeapTracer {
  public:
   /**
    * V8 will call this method at the beginning of the gc cycle.
    */
-  virtual void TraceRoots(Isolate* isolate) = 0;
+  virtual void TracePrologue(Isolate* isolate) = 0;
 
   /**
    * V8 will call this method with internal fields of a potential wrappers.
@@ -5430,14 +5430,14 @@ class EmbedderHeapTracer {
    * PersistentBase::RegisterExternalReference() on all wrappers reachable from
    * any of the given wrappers.
    */
-  virtual void TraceWrappableFrom(
+  virtual void TraceWrappersFrom(
       Isolate* isolate,
       const std::vector<std::pair<void*, void*> >& internal_fields) = 0;
   /**
    * V8 will call this method at the end of the gc cycle. Allocation is *not*
-   * allowed in the ClearTracingMarks.
+   * allowed in the TraceEpilogue.
    */
-  virtual void ClearTracingMarks(Isolate* isolate) = 0;
+  virtual void TraceEpilogue(Isolate* isolate) = 0;
 
  protected:
   virtual ~EmbedderHeapTracer() = default;

src/heap/mark-compact.cc

@@ -2024,7 +2024,7 @@ void MarkCompactCollector::ProcessEphemeralMarking(
   DCHECK(marking_deque_.IsEmpty() && !marking_deque_.overflowed());
   while (work_to_do) {
     if (UsingEmbedderHeapTracer()) {
-      embedder_heap_tracer()->TraceWrappableFrom(
+      embedder_heap_tracer()->TraceWrappersFrom(
           reinterpret_cast<v8::Isolate*>(isolate()), wrappers_to_trace_);
       wrappers_to_trace_.clear();
     } else if (!only_process_harmony_weak_collections) {
@@ -2145,8 +2145,10 @@ void MarkCompactCollector::SetEmbedderHeapTracer(EmbedderHeapTracer* tracer) {
 void MarkCompactCollector::TracePossibleWrapper(JSObject* js_object) {
   DCHECK(js_object->WasConstructedFromApiFunction());
   if (js_object->GetInternalFieldCount() >= 2 &&
+      js_object->GetInternalField(0) &&
       js_object->GetInternalField(0) != heap_->undefined_value() &&
       js_object->GetInternalField(1) != heap_->undefined_value()) {
+    DCHECK(reinterpret_cast<intptr_t>(js_object->GetInternalField(0)) % 2 == 1);
     wrappers_to_trace().push_back(std::pair<void*, void*>(
         reinterpret_cast<void*>(js_object->GetInternalField(0)),
         reinterpret_cast<void*>(js_object->GetInternalField(1))));
@@ -2217,7 +2219,7 @@ void MarkCompactCollector::MarkLiveObjects() {
       TRACE_GC(heap()->tracer(),
                GCTracer::Scope::MC_MARK_WEAK_CLOSURE_EPHEMERAL);
       if (UsingEmbedderHeapTracer()) {
-        embedder_heap_tracer()->TraceRoots(
+        embedder_heap_tracer()->TracePrologue(
             reinterpret_cast<v8::Isolate*>(isolate()));
       }
       ProcessEphemeralMarking(&root_visitor, false);
@@ -2257,7 +2259,7 @@ void MarkCompactCollector::MarkLiveObjects() {
       ProcessEphemeralMarking(&root_visitor, true);
       ProcessMarkingDeque();
       if (UsingEmbedderHeapTracer()) {
-        embedder_heap_tracer()->ClearTracingMarks(
+        embedder_heap_tracer()->TraceEpilogue(
             reinterpret_cast<v8::Isolate*>(isolate()));
       }
     }