Bug 1748032 - Correct, and document, lifetime management of QueueWithSizes elements r=evilpie

Differential Revision: https://phabricator.services.mozilla.com/D135622

dom/streams/QueueWithSizes.h

@@ -27,7 +27,16 @@ struct ValueWithSize : LinkedListElement<ValueWithSize> {
   double mSize = 0.0f;
 };
 
-using QueueWithSizes = LinkedList<ValueWithSize>;
+// This type is a little tricky lifetime wise: Despite the fact that we're
+// talking about linked list of VaueWithSize, what is actually stored in the
+// list are dynamically allocated ValueWithSize*. As a result, these have to be
+// deleted. There are two ways this lifetime is managed:
+//
+// 1. In DequeueValue we pop the first element into a UniquePtr, so that it is
+//    correctly cleaned up at destruction time.
+// 2. We use an AutoCleanLinkedList which will delete elements when destroyed
+//    or `clear`ed.
+using QueueWithSizes = AutoCleanLinkedList<ValueWithSize>;
 
 // https://streams.spec.whatwg.org/#is-non-negative-number
 inline bool IsNonNegativeNumber(double v) {
@@ -59,7 +68,8 @@ inline void EnqueueValueWithSize(QueueContainingClass aContainer,
     return;
   }
 
-  // Step 4.
+  // Step 4. See the comment on QueueWithSizes for the lifetime reasoning
+  //         around this allocation.
   ValueWithSize* valueWithSize = new ValueWithSize(aValue, aSize);
   aContainer->Queue().insertBack(valueWithSize);
   // Step 5.