Implement ShadowRoot.delegatesFocus 2/4 (slide focus on mouse click)

Source/core/input/EventHandler.h

Index: Source/core/input/EventHandler.h
diff --git a/Source/core/input/EventHandler.h b/Source/core/input/EventHandler.h
index eaeb5c42a41aef8dcf1907185af2e425cbfef142..1f6e8c0e2715590451970975ac97e5e98926feb5 100644
--- a/Source/core/input/EventHandler.h
+++ b/Source/core/input/EventHandler.h
@@ -328,6 +328,11 @@ private:
 
     bool shouldTopControlsConsumeScroll(FloatSize) const;
 
+    // If the given element is a shadow host and its root has delegatesFocus=false flag,
+    // slide focus to its inner element. Returns true if the resulting focus is different from
+    // the given element.
+    bool slideFocusOnShadowHostIfNecessary(const Element&);
+
     // NOTE: If adding a new field to this class please ensure that it is
     // cleared in |EventHandler::clear()|.