Measure the usage of prefixed Shadow DOM APIs.

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 fdd23357794cdb6940c255c314abae625e4dde02..1ef78d97a7656dd9e75df2dc7c087b6d496ac213 100644
--- a/Source/core/dom/Element.idl
+++ b/Source/core/dom/Element.idl
@@ -106,9 +106,9 @@
     [EnabledAtRuntime=experimentalShadowDOM, PerWorldBindings] readonly attribute ShadowRoot shadowRoot;
 
     // To-be-deprecated prefixed Shadow DOM API
-    [Reflect=pseudo, ImplementedAs=pseudo, PerWorldBindings] attribute DOMString webkitPseudo;
-    [ImplementedAs=createShadowRoot, RaisesException] ShadowRoot webkitCreateShadowRoot();
-    [ImplementedAs=shadowRoot, PerWorldBindings] readonly attribute ShadowRoot webkitShadowRoot;
+    [Reflect=pseudo, ImplementedAs=pseudo, PerWorldBindings, MeasureAs=ShadowDOMPrefixedPseudo] attribute DOMString webkitPseudo;
+    [ImplementedAs=createShadowRoot, RaisesException, MeasureAs=ShadowDOMPrefixedCreateShadowRoot] ShadowRoot webkitCreateShadowRoot();
+    [ImplementedAs=shadowRoot, PerWorldBindings, MeasureAs=ShadowDOMPrefixedShadowRoot] readonly attribute ShadowRoot webkitShadowRoot;
 
     // CSSOM View Module API
     ClientRectList getClientRects();