[HTML Imports] Implement sub-imports processing.

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index bcc7a0a21c87f8c19372b0d9b196171dbe239f84..a199ab0605daf3ddc5d6a0799bf9c4e7bf7d177c 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -1027,7 +1027,7 @@ public:
     CustomElementRegistry* registry() const { return m_registry.get(); }
     CustomElementRegistry* ensureCustomElementRegistry();
 
-    HTMLImportsController* ensureImports();
+    void setImports(PassRefPtr<HTMLImportsController>);
     HTMLImportsController* imports() const { return m_imports.get(); }
     bool haveImportsLoaded() const;
     void didLoadAllImports();
@@ -1371,7 +1371,7 @@ private:
     OwnPtr<TextAutosizer> m_textAutosizer;
 
     RefPtr<CustomElementRegistry> m_registry;
-    OwnPtr<HTMLImportsController> m_imports;
+    RefPtr<HTMLImportsController> m_imports;
 
     bool m_scheduledTasksAreSuspended;