Make DirectionGranularityStrategy::UpdateExtent() to check null position for vertical movement

This patch makes |DirectionGranularityStrategy::UpdateExtent()| to check null
position fo vertical movement to prevent constructing invalid selection for
avoiding crash.

This patch is similar to the CL[1], but we are not sure actual HTML causes
this crash. This CL is an experimental to reduce candidates of crash.

[1] http://crrev.com/c/1669133 Make DirectionGranularityStrategy::UpdateExtent()
to check null position for word bounds

Bug: 978784
Change-Id: Ica41e0dd75c0ded2beafa171f1d027bf5de9cae5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775956
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691950}

third_party/blink/renderer/core/editing/granularity_strategy.cc

@@ -148,6 +148,8 @@ SelectionInDOMTree DirectionGranularityStrategy::UpdateExtent(
     new_offset_extent_point = extent_point;
     new_offset_extent_position = CreateVisiblePosition(
         PositionForContentsPointRespectingEditingBoundary(extent_point, frame));
+    if (new_offset_extent_position.IsNull())
+      return selection.AsSelection();
   }
 
   const VisiblePosition base = selection.VisibleBase();