Bug 1684965 - [devtools] Manage a reference to the parent in Pool. r=ochameau.

At the moment, the `getParent` method was using the connection `poolFor` method
to retrieve the pool that was managing it. This is quite costly as `poolFor`
loops through all the pools of the connection.
This patch adds a `parentPool` property to the Pool that is set in `manage` and
reset in `unmanage`, and used in the `getParent` method.
This speeds up getParent as well as the methods that call it (`manage` when the
actor was already managed, and `destroy`).

Differential Revision: https://phabricator.services.mozilla.com/D100981

devtools/shared/protocol/Pool.js

@@ -30,14 +30,16 @@ class Pool extends EventEmitter {
       this.conn = conn;
     }
     this.label = label;
-    this.__poolMap = null;
   }
 
+  __poolMap = null;
+  parentPool = null;
+
   /**
    * Return the parent pool for this client.
    */
   getParent() {
-    return this.conn.poolFor(this.actorID);
+    return this.parentPool;
   }
 
   /**
@@ -93,6 +95,7 @@ class Pool extends EventEmitter {
       }
     }
     this._poolMap.set(actor.actorID, actor);
+    actor.parentPool = this;
   }
 
   unmanageChildren(FrontType) {
@@ -107,7 +110,11 @@ class Pool extends EventEmitter {
    * Remove an actor as a child of this pool.
    */
   unmanage(actor) {
-    this.__poolMap && this.__poolMap.delete(actor.actorID);
+    if (!this.__poolMap) {
+      return;
+    }
+    this.__poolMap.delete(actor.actorID);
+    actor.parentPool = null;
   }
 
   // true if the given actor ID exists in the pool.