Make FrameSelection::Contains return false for null VisibleSelection in DOM tree

This is a workaround for crbug.com/845849.

In some buggy cases, we get ComputeVisibleSelectionInFlatTree() returning
non-null, but ComputeVisibleSelectionInDOMTree() returning null. This
makes FrameSelection::Contains() return true erroneously.

As we don't have a fix to the inconsistency, this patch adds a
workaround that, we call ComputeVisibleSelectionInDOMTree() first to
set the cached VisibleSelections to null also for flat tree, so that
the following computation does not work on a non-null VS in flat tree
erroneously.

Bug: 845849
Change-Id: I7255d8224b8b6e29d479dd635e2c4ee99b366fc1
Reviewed-on: https://chromium-review.googlesource.com/1072924Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561775}

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

@@ -554,6 +554,14 @@ bool FrameSelection::Contains(const LayoutPoint& point) {
   if (!GetDocument().GetLayoutView())
     return false;
 
+  // This is a workaround of the issue that we sometimes get null from
+  // ComputeVisibleSelectionInDOMTree(), but non-null from flat tree.
+  // By running this, in case we get null, we also set the cached result in flat
+  // tree into null, so that this function can return false correctly.
+  // See crbug.com/846527 for details.
+  // TODO(editing-dev): Fix the inconsistency and then remove this call.
+  ComputeVisibleSelectionInDOMTree();
+
   // Treat a collapsed selection like no selection.
   const VisibleSelectionInFlatTree& visible_selection =
       ComputeVisibleSelectionInFlatTree();