Have Document::timeline() / Document::transitionTimeline() return a reference

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 92f5c6dd47b0fc04183972ac590261bdfc925442..4ade4b697f02299d1b25e09be75ff58ab4c061ca 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -547,13 +547,8 @@ Document::~Document()
         m_import = 0;
     }
 
-    if (m_timeline) {
-        m_timeline->detachFromDocument();
-    }
-
-    if (m_transitionTimeline) {
-        m_transitionTimeline->detachFromDocument();
-    }
+    m_timeline->detachFromDocument();
+    m_transitionTimeline->detachFromDocument();
 
     m_styleEngine.clear(); // We need to destory CSSFontSelector before destroying m_fetcher.