[net] Better StopCaching() handling for HttpCache::Transaction.

net/url_request/url_request.h

Index: net/url_request/url_request.h
diff --git a/net/url_request/url_request.h b/net/url_request/url_request.h
index c6ef160e69c4fb2ace304bc27e9af0cbe6c972a9..19ba29a9134ce2f225a8c1b557a799c136b1f64b 100644
--- a/net/url_request/url_request.h
+++ b/net/url_request/url_request.h
@@ -541,6 +541,8 @@ class NET_EXPORT URLRequest : NON_EXPORTED_BASE(public base::NonThreadSafe),
   // for the same resource. The typical example is a request that results in
   // the data being stored to disk (downloaded instead of rendered) so we don't
   // want to store it twice.
+  //
+  // Should only be called if there are no pending operations.
   void StopCaching();
 
   // This method may be called to follow a redirect that was deferred in