Introduce "navigation target classification" for spatial navigation

Source/core/page/SpatialNavigation.h

Index: Source/core/page/SpatialNavigation.h
diff --git a/Source/core/page/SpatialNavigation.h b/Source/core/page/SpatialNavigation.h
index c4a5cfb46bce776ced83383ade66fd175b5e0d86..779354d954b534d0f60f4fe545a25a38ac7ab078 100644
--- a/Source/core/page/SpatialNavigation.h
+++ b/Source/core/page/SpatialNavigation.h
@@ -94,6 +94,16 @@ enum RectsAlignment {
     Full
 };
 
+// Classification of a focus candidate, ie. why a candidate is considered
+// "interesting" as a navigation target.
+enum NavigationClass {
+    NavigationClassNone,
+    NavigationClassFocusable,
+    NavigationClassFrameOwner,
+    NavigationClassScrollable,
+    NavigationClassEventHandler,
+};
+
 struct FocusCandidate {
     STACK_ALLOCATED();
 public:
@@ -105,11 +115,11 @@ public:
         , alignment(None)
         , isOffscreen(true)
         , isOffscreenAfterScrolling(true)
+        , targetClass(NavigationClassNone)
     {
     }
 
-    FocusCandidate(Node*, FocusType);
-    explicit FocusCandidate(HTMLAreaElement*, FocusType);
+    FocusCandidate(Node*, FocusType, NavigationClass);
     bool isNull() const { return !visibleNode; }
     bool inScrollableContainer() const { return visibleNode && enclosingScrollableBox; }
     bool isFrameOwnerElement() const { return visibleNode && visibleNode->isFrameOwnerElement(); }
@@ -126,6 +136,7 @@ public:
     LayoutRect rect;
     bool isOffscreen;
     bool isOffscreenAfterScrolling;
+    NavigationClass targetClass;
 };
 
 bool hasOffscreenRect(Node*, FocusType = FocusTypeNone);