Revert of Re-landing r170008, removing prefixed Shadow DOM APIs in Element.

Source/core/dom/Element.idl

Index: Source/core/dom/Element.idl
diff --git a/Source/core/dom/Element.idl b/Source/core/dom/Element.idl
index c6645c8317308d44fa3b9412403bd02c0e50a00e..cce1359ffc1af4f036f2197652941dff9640318b 100644
--- a/Source/core/dom/Element.idl
+++ b/Source/core/dom/Element.idl
@@ -115,6 +115,10 @@
     [PerWorldBindings] readonly attribute ShadowRoot shadowRoot;
     [PerWorldBindings] NodeList getDestinationInsertionPoints();
 
+    // To-be-deprecated prefixed Shadow DOM API
+    [ImplementedAs=createShadowRoot, RaisesException, MeasureAs=ShadowDOMPrefixedCreateShadowRoot] ShadowRoot webkitCreateShadowRoot();
+    [ImplementedAs=shadowRoot, PerWorldBindings, MeasureAs=ShadowDOMPrefixedShadowRoot] readonly attribute ShadowRoot webkitShadowRoot;
+
     // CSSOM View Module API
     ClientRectList getClientRects();
     ClientRect getBoundingClientRect();