Don't clamp scroll positions until the end of all layouts

third_party/WebKit/Source/core/layout/LayoutBlock.h

Index: third_party/WebKit/Source/core/layout/LayoutBlock.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutBlock.h b/third_party/WebKit/Source/core/layout/LayoutBlock.h
index c621c78145195ca30c53d77ea6996935f1e84276..9976558c884dca6e16369916f62c3515e8ddbcbd 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBlock.h
+++ b/third_party/WebKit/Source/core/layout/LayoutBlock.h
@@ -341,8 +341,12 @@ protected:
     // finishDelayUpdateScrollInfo() will do nothing until finishDelayUpdateScrollInfo()
     // is called the same number of times.
     // finishDelayUpdateScrollInfo returns true when it marked something for layout.
+    // It will also return a map of saved scroll positions that the caller should restore
+    // on the given scrollable areas after performing the layout.
+    // This can be necessary because Flexbox's multi-pass layout can lose the scroll position.
+    typedef WTF::HashMap<PaintLayerScrollableArea*, DoublePoint> ScrollPositionMap;
     static void startDelayUpdateScrollInfo();
-    static bool finishDelayUpdateScrollInfo(SubtreeLayoutScope*);
+    static bool finishDelayUpdateScrollInfo(SubtreeLayoutScope*, ScrollPositionMap*);
 
     void updateScrollInfoAfterLayout();