Bug 1594181 - Reset external variable properly on destruction. r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D52639

--HG--
extra : moz-landing-system : lando

dom/media/mediacapabilities/BenchmarkStorageChild.cpp

@@ -24,4 +24,11 @@ PBenchmarkStorageChild* BenchmarkStorageChild::Instance() {
   return sChild;
 }
 
+BenchmarkStorageChild::~BenchmarkStorageChild() {
+  MOZ_ASSERT(NS_IsMainThread());
+  if (sChild == this) {
+    sChild = nullptr;
+  }
+}
+
 }  // namespace mozilla

dom/media/mediacapabilities/BenchmarkStorageChild.h

@@ -17,6 +17,7 @@ class BenchmarkStorageChild : public PBenchmarkStorageChild {
    * to the storage. */
   static PBenchmarkStorageChild* Instance();
 
+  ~BenchmarkStorageChild();
  private:
   BenchmarkStorageChild() = default;
 };