diff -r 000000000000 -r 6474c204b198 layout/base/nsIPercentHeightObserver.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/base/nsIPercentHeightObserver.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,30 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsIPercentHeightObserver_h___
+#define nsIPercentHeightObserver_h___
+
+#include "nsQueryFrame.h"
+
+struct nsHTMLReflowState;
+
+/**
+ * This interface is supported by frames that need to provide computed height
+ * values to children during reflow which would otherwise not happen. Currently only
+ * table cells support this.
+ */
+class nsIPercentHeightObserver
+{
+public:
+  NS_DECL_QUERYFRAME_TARGET(nsIPercentHeightObserver)
+
+  // Notify the observer that aReflowState has no computed height, but it has a percent height
+  virtual void NotifyPercentHeight(const nsHTMLReflowState& aReflowState) = 0;
+
+  // Ask the observer if it should observe aReflowState.frame
+  virtual bool NeedsToObserve(const nsHTMLReflowState& aReflowState) = 0;
+};
+
+#endif // nsIPercentHeightObserver_h___