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

Source/core/editing/VisibleUnits.cpp

Index: Source/core/editing/VisibleUnits.cpp
diff --git a/Source/core/editing/VisibleUnits.cpp b/Source/core/editing/VisibleUnits.cpp
index 7ad6c8803304fd079760da388d6720a9a9846c3e..fa94d0c52f8e2f39ed369bb9a680fbc5d830820c 100644
--- a/Source/core/editing/VisibleUnits.cpp
+++ b/Source/core/editing/VisibleUnits.cpp
@@ -1329,7 +1329,7 @@ bool isEndOfBlock(const VisiblePosition &pos)
 
 VisiblePosition startOfDocument(const Node* node)
 {
-    if (!node || !node->document() || !node->document()->documentElement())
+    if (!node || !node->document()->documentElement())
         return VisiblePosition();
 
     return VisiblePosition(firstPositionInNode(node->document()->documentElement()), DOWNSTREAM);
@@ -1342,7 +1342,7 @@ VisiblePosition startOfDocument(const VisiblePosition &c)
 
 VisiblePosition endOfDocument(const Node* node)
 {
-    if (!node || !node->document() || !node->document()->documentElement())
+    if (!node || !node->document()->documentElement())
         return VisiblePosition();
 
     Element* doc = node->document()->documentElement();