Add clean layout DCHECK to mostForwardCaretPosition()

third_party/WebKit/Source/core/editing/VisibleUnits.cpp

Index: third_party/WebKit/Source/core/editing/VisibleUnits.cpp
diff --git a/third_party/WebKit/Source/core/editing/VisibleUnits.cpp b/third_party/WebKit/Source/core/editing/VisibleUnits.cpp
index d14f4be68350a5fc4d2e59c7b57edb9bdd8420d2..65847db73f0d647212c24038a5e93fe3d61bc4cd 100644
--- a/third_party/WebKit/Source/core/editing/VisibleUnits.cpp
+++ b/third_party/WebKit/Source/core/editing/VisibleUnits.cpp
@@ -3018,6 +3018,7 @@ template <typename Strategy>
 PositionTemplate<Strategy> mostForwardCaretPosition(
     const PositionTemplate<Strategy>& position,
     EditingBoundaryCrossingRule rule) {
+  DCHECK(!needsLayoutTreeUpdate(position)) << position;
   TRACE_EVENT0("input", "VisibleUnits::mostForwardCaretPosition");
 
   Node* startNode = position.anchorNode();