[HTML Imports] Implement sub-imports processing.

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 5b418e7eb4c88c75962bdfda1f2b497207aa2865..6c45bfab388851b16f10c17fa2fe743e9c09c5a0 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -856,11 +856,10 @@ CustomElementRegistry* Document::ensureCustomElementRegistry()
     return m_registry.get();
 }
 
-HTMLImportsController* Document::ensureImports()
+void Document::setImports(PassRefPtr<HTMLImportsController> imports)
 {
-    if (!m_imports)
-        m_imports = HTMLImportsController::create(this);
-    return m_imports.get();
+    ASSERT(!m_imports);
+    m_imports = imports;
 }
 
 void Document::didLoadAllImports()