Bug 1735970 part 7: Make AccIterable::Next return an Accessible. r=morgan

We need to be able to iterate through explicitly associated headers for both local and remote Accessibles.
AccIterable will serve nicely as a base class, but it needs to support the Accessible base class to do that.

Differential Revision: https://phabricator.services.mozilla.com/D141210

accessible/base/AccIterator.h

@@ -23,7 +23,7 @@ namespace a11y {
 class AccIterable {
  public:
   virtual ~AccIterable() {}
-  virtual LocalAccessible* Next() = 0;
+  virtual Accessible* Next() = 0;
 
  private:
   friend class Relation;

accessible/base/Relation.h

@@ -75,7 +75,7 @@ class Relation {
    * compute and return the next related accessible.
    */
   inline LocalAccessible* Next() {
-    LocalAccessible* target = nullptr;
+    Accessible* target = nullptr;
 
     while (mFirstIter && !(target = mFirstIter->Next())) {
       mFirstIter = std::move(mFirstIter->mNextIter);
@@ -83,7 +83,7 @@ class Relation {
 
     if (!mFirstIter) mLastIter = nullptr;
 
-    return target;
+    return target ? target->AsLocal() : nullptr;
   }
 
  private: