Bug fix: internals.textSurroundingNode crashes when the node is NULL

Added NULL check.

BUG=354958
TEST=Check there is no crashes when the minimized test case is passed to
content_shell

Review URL: https://codereview.chromium.org/201573008

git-svn-id: svn://svn.chromium.org/blink/trunk@169849 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/testing/Internals.cpp

@@ -2444,6 +2444,8 @@ void Internals::setValueForUser(Element* element, const String& value)
 
 String Internals::textSurroundingNode(Node* node, int x, int y, unsigned long maxLength)
 {
+    if (!node)
+        return String();
     blink::WebPoint point(x, y);
     SurroundingText surroundingText(VisiblePosition(node->renderer()->positionForPoint(static_cast<IntPoint>(point))), maxLength);
     return surroundingText.content();