Remove synchronization from Http2Connection.ExtendWindow (#97878)

_pendingWindowUpdate is the only state the ExtendWindow method touches under the lock. There is no other code that reads or writes _pendingWindowUpdate after the creation of the connection object. Originating from ProcessIncomingFramesAsync, calls to ExtendWindow are sequential, and never expected to happen concurrently.

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Connection.cs

@@ -1771,23 +1771,18 @@ private bool ExtendWindow(int amount)
         {
             if (NetEventSource.Log.IsEnabled()) Trace($"{nameof(amount)}={amount}");
             Debug.Assert(amount > 0);
+            Debug.Assert(_pendingWindowUpdate < ConnectionWindowThreshold);
 
-            int windowUpdateSize;
-            lock (SyncObject)
+            _pendingWindowUpdate += amount;
+            if (_pendingWindowUpdate < ConnectionWindowThreshold)
             {
-                Debug.Assert(_pendingWindowUpdate < ConnectionWindowThreshold);
-
-                _pendingWindowUpdate += amount;
-                if (_pendingWindowUpdate < ConnectionWindowThreshold)
-                {
-                    if (NetEventSource.Log.IsEnabled()) Trace($"{nameof(_pendingWindowUpdate)} {_pendingWindowUpdate} < {ConnectionWindowThreshold}.");
-                    return false;
-                }
-
-                windowUpdateSize = _pendingWindowUpdate;
-                _pendingWindowUpdate = 0;
+                if (NetEventSource.Log.IsEnabled()) Trace($"{nameof(_pendingWindowUpdate)} {_pendingWindowUpdate} < {ConnectionWindowThreshold}.");
+                return false;
             }
 
+            int windowUpdateSize = _pendingWindowUpdate;
+            _pendingWindowUpdate = 0;
+
             LogExceptions(SendWindowUpdateAsync(0, windowUpdateSize));
             return true;
         }