Have Node::document() return a reference instead of a pointer

Source/core/html/HTMLTrackElement.cpp

Index: Source/core/html/HTMLTrackElement.cpp
diff --git a/Source/core/html/HTMLTrackElement.cpp b/Source/core/html/HTMLTrackElement.cpp
index 100648406a01c09d33087bffc186cafed176f846..4b55f78ff52e4ccc999aa3e8d520db91ecbfb5d6 100644
--- a/Source/core/html/HTMLTrackElement.cpp
+++ b/Source/core/html/HTMLTrackElement.cpp
@@ -118,7 +118,7 @@ void HTMLTrackElement::parseAttribute(const QualifiedName& name, const AtomicStr
 
 KURL HTMLTrackElement::src() const
 {
-    return document()->completeURL(getAttribute(srcAttr));
+    return document().completeURL(getAttribute(srcAttr));
 }
 
 void HTMLTrackElement::setSrc(const String& url)
@@ -252,7 +252,7 @@ bool HTMLTrackElement::canLoadUrl(const KURL& url)
     if (url.isEmpty())
         return false;
 
-    if (!document()->contentSecurityPolicy()->allowMediaFromSource(url)) {
+    if (!document().contentSecurityPolicy()->allowMediaFromSource(url)) {
         LOG(Media, "HTMLTrackElement::canLoadUrl(%s) -> rejected by Content Security Policy", urlForLoggingTrack(url).utf8().data());
         return false;
     }