Bug 1623715 - [4.1] Fix LinkedList::length() for a const context. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D84543

dom/media/mediacontrol/MediaControlService.cpp

@@ -450,14 +450,7 @@ MediaController* MediaControlService::ControllerManager::GetMainController()
 }
 
 uint64_t MediaControlService::ControllerManager::GetControllersNum() const {
-  size_t length = 0;
-  const auto* element =
-      static_cast<ConstLinkedListControllerPtr>(mControllers.getFirst());
-  while (element) {
-    length++;
-    element = element->getNext();
-  }
-  return length;
+  return mControllers.length();
 }
 
 bool MediaControlService::ControllerManager::Contains(

mfbt/LinkedList.h

@@ -515,15 +515,7 @@ class LinkedList {
   /**
    * Return the length of elements in the list.
    */
-  size_t length() const {
-    size_t length = 0;
-    ConstRawType element = getFirst();
-    while (element) {
-      length++;
-      element = element->getNext();
-    }
-    return length;
-  }
+  size_t length() const { return std::distance(begin(), end()); }
 
   /*
    * Allow range-based iteration: