Polish EmbedderHeapTracer and move some checks from blink to v8

Quick one, ptal.

BUG=468240
LOG=no

Committed: https://crrev.com/0eae5650dec265e961a1024fd2090301f2cdca16
Cr-Commit-Position: refs/heads/master@{#35578}

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

Cr-Commit-Position: refs/heads/master@{#35594}

src/heap/mark-compact.cc

@@ -2148,7 +2148,7 @@ void MarkCompactCollector::TracePossibleWrapper(JSObject* js_object) {
       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);
+    DCHECK(reinterpret_cast<intptr_t>(js_object->GetInternalField(0)) % 2 == 0);
     wrappers_to_trace().push_back(std::pair<void*, void*>(
         reinterpret_cast<void*>(js_object->GetInternalField(0)),
         reinterpret_cast<void*>(js_object->GetInternalField(1))));