Bug 1660555 - Add a |Signal()| accessor to |AbortFollower| rather than directly accessing an inherited, protected member. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D93888

dom/abort/AbortSignal.h

@@ -31,6 +31,8 @@ class AbortFollower {
 
   bool IsFollowing() const;
 
+  AbortSignalImpl* Signal() const { return mFollowingSignal; }
+
  protected:
   virtual ~AbortFollower();
 

dom/fetch/Fetch.cpp

@@ -166,7 +166,10 @@ class AbortSignalProxy final : public AbortFollower {
     return mSignalImplMainThread;
   }
 
-  AbortSignalImpl* GetSignalImplForTargetThread() { return mFollowingSignal; }
+  AbortSignalImpl* GetSignalImplForTargetThread() {
+    MOZ_ASSERT(!NS_IsMainThread());
+    return Signal();
+  }
 
   void Shutdown() { Unfollow(); }