simplify generation of STREAM_DATA_BLOCKED frames (#4608)

The stream always gets blocked at the current write offset. There's no
need to return this offset from the flow controller.

internal/flowcontrol/interface.go

@@ -9,7 +9,6 @@ type flowController interface {
 	AddBytesSent(protocol.ByteCount)
 	// for receiving
 	GetWindowUpdate() protocol.ByteCount // returns 0 if no update is necessary
-	IsNewlyBlocked() (bool, protocol.ByteCount)
 }
 
 // A StreamFlowController is a flow controller for a QUIC stream.
@@ -23,6 +22,7 @@ type StreamFlowController interface {
 	// Abandon is called when reading from the stream is aborted early,
 	// and there won't be any further calls to AddBytesRead.
 	Abandon()
+	IsNewlyBlocked() bool
 }
 
 // The ConnectionFlowController is the flow controller for the connection.
@@ -30,6 +30,7 @@ type ConnectionFlowController interface {
 	flowController
 	AddBytesRead(protocol.ByteCount)
 	Reset() error
+	IsNewlyBlocked() (bool, protocol.ByteCount)
 }
 
 type connectionFlowControllerI interface {

internal/flowcontrol/stream_flow_controller.go

@@ -121,6 +121,11 @@ func (c *streamFlowController) SendWindowSize() protocol.ByteCount {
 	return min(c.baseFlowController.sendWindowSize(), c.connection.SendWindowSize())
 }
 
+func (c *streamFlowController) IsNewlyBlocked() bool {
+	blocked, _ := c.baseFlowController.IsNewlyBlocked()
+	return blocked
+}
+
 func (c *streamFlowController) shouldQueueWindowUpdate() bool {
 	return !c.receivedFinalOffset && c.hasWindowUpdate()
 }