Fix crash on GestureTap in Node::commonAncestor

LayoutTests/fast/events/touch/gesture/gesture-tap-frame-scrollbar-expected.txt

Index: LayoutTests/fast/events/touch/gesture/gesture-tap-frame-scrollbar-expected.txt
diff --git a/LayoutTests/fast/events/touch/gesture/gesture-tap-frame-scrollbar-expected.txt b/LayoutTests/fast/events/touch/gesture/gesture-tap-frame-scrollbar-expected.txt
new file mode 100644
index 0000000000000000000000000000000000000000..180ec33759bfdb07a2823cee75c78db4a5a9709b
--- /dev/null
+++ b/LayoutTests/fast/events/touch/gesture/gesture-tap-frame-scrollbar-expected.txt
@@ -0,0 +1,32 @@
+
+Taps at a point that becomes a scrollbar (i.e. hit test with innerNode=null) during the tap and verifies we don't crash.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+Test case: Tap on consistent scrollbar
+Sending GestureTap
+
+Test case: Add scrollbars during mousemove
+Sending GestureTap
+Received mousemove in child frame
+Adding scrollbars to iframe
+
+Test case: Add scrollbars during mousedown
+Sending GestureTap
+Received mousemove in child frame
+Received mousedown in child frame
+Adding scrollbars to iframe
+
+Test case: Add scrollbars during mouseup
+Sending GestureTap
+Received mousemove in child frame
+Received mousedown in child frame
+Received mouseup in child frame
+Adding scrollbars to iframe
+Received click in child frame
+
+PASS successfullyParsed is true
+
+TEST COMPLETE
+