Selection API: Fix a null pointer dereference in addRange().

third_party/WebKit/Source/core/editing/DOMSelection.cpp

Index: third_party/WebKit/Source/core/editing/DOMSelection.cpp
diff --git a/third_party/WebKit/Source/core/editing/DOMSelection.cpp b/third_party/WebKit/Source/core/editing/DOMSelection.cpp
index 9d5c79129f8a36f0c44ffb284c4f318fa609f68a..1e55462980a5f9507a0a91940d4d5350cd9008d8 100644
--- a/third_party/WebKit/Source/core/editing/DOMSelection.cpp
+++ b/third_party/WebKit/Source/core/editing/DOMSelection.cpp
@@ -617,7 +617,8 @@ void DOMSelection::addRange(Range* newRange) {
     return;
   }
 
-  Range* originalRange = selection.firstRange();
+  Range* originalRange = primaryRangeOrNull();
+  DCHECK(originalRange);
 
   if (originalRange->startContainer()->treeScope() !=
       newRange->startContainer()->treeScope()) {