Use phantom handle in GCObservation

The GCObservation handle is always set and empty state implies that the
observed object was garbage collection. Use phantom reference which
avoids clearing out the reference manually.

Bug: 903586, 843903
Change-Id: I77e6e9d9ef4625d4bb91b3907b0d1b69c17f2208
Reviewed-on: https://chromium-review.googlesource.com/c/1340320Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608771}

third_party/blink/renderer/core/testing/gc_observation.cc

@@ -32,20 +32,10 @@
 
 namespace blink {
 
-static void SetWeakCallbackForGCObservation(
-    const v8::WeakCallbackInfo<GCObservation>& data) {
-  data.GetParameter()->SetWasCollected();
-}
-
-void GCObservation::SetWasCollected() {
-  DCHECK(!wasCollected());
-  observed_.Clear();
-  collected_ = true;
-}
-
 GCObservation::GCObservation(v8::Local<v8::Value> observed_value)
-    : observed_(v8::Isolate::GetCurrent(), observed_value), collected_(false) {
-  observed_.SetWeak(this, SetWeakCallbackForGCObservation);
+    : observed_(v8::Isolate::GetCurrent(), observed_value) {
+  CHECK(!wasCollected());
+  observed_.SetPhantom();
 }
 
 }  // namespace blink

third_party/blink/renderer/core/testing/gc_observation.h

@@ -50,14 +50,12 @@ class GCObservation final : public ScriptWrappable {
   // "near death"; it may have been kept alive through a weak
   // handle. After reaching near-death, having been collected is the
   // common case.
-  bool wasCollected() const { return collected_; }
-  void SetWasCollected();
+  bool wasCollected() const { return observed_.IsEmpty(); }
 
  private:
   explicit GCObservation(v8::Local<v8::Value>);
 
   ScopedPersistent<v8::Value> observed_;
-  bool collected_;
 };
 
 }  // namespace blink