Make toPositionInFlatTree() to handle a position anchored by shadow root

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

Index: third_party/WebKit/Source/core/editing/Position.cpp
diff --git a/third_party/WebKit/Source/core/editing/Position.cpp b/third_party/WebKit/Source/core/editing/Position.cpp
index 01f0364a006e652f08bdf32c1affcd23bf5451aa..c0a6bf2259c17821451675d374f728b81f9839a9 100644
--- a/third_party/WebKit/Source/core/editing/Position.cpp
+++ b/third_party/WebKit/Source/core/editing/Position.cpp
@@ -513,8 +513,8 @@ PositionInFlatTree toPositionInFlatTree(const Position& pos)
     if (pos.isNull())
         return PositionInFlatTree();
 
+    Node* const anchor = pos.anchorNode();
     if (pos.isOffsetInAnchor()) {
-        Node* anchor = pos.anchorNode();
         if (anchor->isCharacterDataNode())
             return PositionInFlatTree(anchor, pos.computeOffsetInContainerNode());
         DCHECK(!anchor->isSlotOrActiveInsertionPoint());
@@ -541,7 +541,11 @@ PositionInFlatTree toPositionInFlatTree(const Position& pos)
         return PositionInFlatTree(anchor, PositionAnchorType::AfterChildren);
     }
 
-    return PositionInFlatTree(pos.anchorNode(), pos.anchorType());
+    if (anchor->isShadowRoot())
+        return PositionInFlatTree(anchor->shadowHost(), pos.anchorType());
+    // TODO(yosin): Once we have a test case for SLOT or active insertion point,
+    // this function should handle it.
+    return PositionInFlatTree(anchor, pos.anchorType());
 }
 
 Position toPositionInDOMTree(const Position& position)