Remove useless null checks from Node::document() call sites

Source/core/accessibility/AccessibilityObject.cpp

Index: Source/core/accessibility/AccessibilityObject.cpp
diff --git a/Source/core/accessibility/AccessibilityObject.cpp b/Source/core/accessibility/AccessibilityObject.cpp
index f1ee094565411656c32eb6766135275b6a230c2f..e474e0fe49d4c41b882d45540c39bab4a1513fc4 100644
--- a/Source/core/accessibility/AccessibilityObject.cpp
+++ b/Source/core/accessibility/AccessibilityObject.cpp
@@ -505,12 +505,7 @@ AccessibilityObject* AccessibilityObject::firstAccessibleObjectFromNode(const No
     if (!node)
         return 0;
 
-    Document* document = node->document();
-    if (!document)
-        return 0;
-
-    AXObjectCache* cache = document->axObjectCache();
-
+    AXObjectCache* cache = node->document()->axObjectCache();
     AccessibilityObject* accessibleObject = cache->getOrCreate(node->renderer());
     while (accessibleObject && accessibleObject->accessibilityIsIgnored()) {
         node = NodeTraversal::next(node);