Made Text#assignedSlot and Element#assignedSlot nullable.

third_party/WebKit/Source/core/dom/Text.idl

Index: third_party/WebKit/Source/core/dom/Text.idl
diff --git a/third_party/WebKit/Source/core/dom/Text.idl b/third_party/WebKit/Source/core/dom/Text.idl
index 74cfdac6547a9a43ccf3bf0a9e1e1ff4aa6c7687..0ed45a81c9669fe28393500dc70d5217e8020a3b 100644
--- a/third_party/WebKit/Source/core/dom/Text.idl
+++ b/third_party/WebKit/Source/core/dom/Text.idl
@@ -26,8 +26,10 @@
     [NewObject, DoNotTestNewObject, RaisesException] Text splitText(unsigned long offset);
     [MeasureAs=TextWholeText] readonly attribute DOMString wholeText;
 
-    // Shadow DOM
-    // https://w3c.github.io/webcomponents/spec/shadow/#extensions-to-text-interface
+    // Mixin Slotable
+    // https://dom.spec.whatwg.org/#mixin-slotable
+    [ImplementedAs=assignedSlotForBinding] readonly attribute HTMLSlotElement? assignedSlot;
+
+    // Non-standard API:
     NodeList getDestinationInsertionPoints();
-    [ImplementedAs=assignedSlotForBinding] readonly attribute HTMLSlotElement assignedSlot;
 };