Add a function that calculates the size of WebSocket frame header.

net/websockets/websocket_frame.h

Index: net/websockets/websocket_frame.h
diff --git a/net/websockets/websocket_frame.h b/net/websockets/websocket_frame.h
index 9f7c7fd6fc26b471fd38cc8db84a7f2f95ea9068..94f4ede680254dde32a0ba2f163531acf401b915 100644
--- a/net/websockets/websocket_frame.h
+++ b/net/websockets/websocket_frame.h
@@ -86,6 +86,12 @@ struct WebSocketMaskingKey {
   char key[WebSocketFrameHeader::kMaskingKeyLength];
 };
 
+// Returns the size of WebSocket frame header. The size of WebSocket frame
+// header varies from 2 bytes to 14 bytes depending on the payload length
+// and maskedness.
+NET_EXPORT_PRIVATE int GetWebSocketFrameHeaderSize(
+    const WebSocketFrameHeader& header);
+
 // Writes wire format of a WebSocket frame header into |output|, and returns
 // the number of bytes written.
 //
@@ -98,10 +104,10 @@ struct WebSocketMaskingKey {
 // (possibly generated by GenerateWebSocketMaskingKey() function below).
 // Otherwise, |masking_key| must be NULL.
 //
-// |buffer| should have enough size to contain the frame header. Size of a
-// frame header varies from 2 bytes to 14 bytes depending on the payload length
-// and maskedness. If the size of |buffer| is insufficient, this function
-// returns ERR_INVALID_ARGUMENT and does not write any data to |buffer|.
+// |buffer| should have enough size to contain the frame header.
+// GetWebSocketFrameHeaderSize() can be used to know the size of header
+// beforehand. If the size of |buffer| is insufficient, this function returns
+// ERR_INVALID_ARGUMENT and does not write any data to |buffer|.
 NET_EXPORT_PRIVATE int WriteWebSocketFrameHeader(
     const WebSocketFrameHeader& header,
     const WebSocketMaskingKey* masking_key,