Refactor setImportsController() to clarify invariants

third_party/WebKit/Source/core/dom/Document.cpp

Index: third_party/WebKit/Source/core/dom/Document.cpp
diff --git a/third_party/WebKit/Source/core/dom/Document.cpp b/third_party/WebKit/Source/core/dom/Document.cpp
index dd47f2aa3a69bae0508d3521d70c7ca0cc2cefc1..e2ffcd3d20a2427deb069f79eb4b543818d453ce 100644
--- a/third_party/WebKit/Source/core/dom/Document.cpp
+++ b/third_party/WebKit/Source/core/dom/Document.cpp
@@ -904,13 +904,17 @@ V0CustomElementMicrotaskRunQueue* Document::customElementMicrotaskRunQueue() {
   return m_customElementMicrotaskRunQueue.get();
 }
 
-void Document::setImportsController(HTMLImportsController* controller) {
-  DCHECK(!m_importsController || !controller);
-  m_importsController = controller;
-  if (!m_importsController && !loader())
+void Document::clearImportsController() {
+  m_importsController = nullptr;
+  if (!loader())
     m_fetcher->clearContext();
 }
 
+void Document::createImportsController() {
+  DCHECK(!m_importsController);
+  m_importsController = HTMLImportsController::create(*this);
+}
+
 HTMLImportLoader* Document::importLoader() const {
   if (!m_importsController)
     return 0;
@@ -2480,7 +2484,7 @@ void Document::shutdown() {
   // thinking they should have access to a valid frame when they don't.
   if (m_importsController) {
     m_importsController->dispose();
-    setImportsController(nullptr);
+    clearImportsController();
   }
 
   m_timers.setTimerTaskRunner(Platform::current()