Make ComputePositionForNodeRemoval() to work correctly for after node/children position

This patch changes |ComputePositionForNodeRemoval()| to work correctly
for after node/children position to make |SelectionEditor| relocates
base and extent positions correctly.

Before this patch |ComputePositionForNodeRemoval()| returns position
after removed node as child index in parent, e.g. parent@child+1.
After this patch |ComputePositionForNodeRemoval()| returns parent@child.

Example:
<div><a></a><b></b></div>
remove <a>
<div><b></b></div>

When removing <a>, position after <a> should be relocated to <div>@0.

Bug: 1145595
Change-Id: Ibd5f30b52cb10ba066dfeca3fbf9f1d5bef6c77b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558123
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830898}

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

@@ -1405,7 +1405,7 @@ Position ComputePositionForNodeRemoval(const Position& position,
           !node.IsShadowIncludingInclusiveAncestorOf(*container_node)) {
         return position;
       }
-      return Position::InParentAfterNode(node);
+      return Position::InParentBeforeNode(node);
     case PositionAnchorType::kOffsetInAnchor:
       container_node = position.ComputeContainerNode();
       if (container_node == node.parentNode() &&
@@ -1423,7 +1423,7 @@ Position ComputePositionForNodeRemoval(const Position& position,
       if (!anchor_node ||
           !node.IsShadowIncludingInclusiveAncestorOf(*anchor_node))
         return position;
-      return Position::InParentAfterNode(node);
+      return Position::InParentBeforeNode(node);
     case PositionAnchorType::kBeforeAnchor:
       anchor_node = position.AnchorNode();
       if (!anchor_node ||

third_party/blink/renderer/core/editing/editing_utilities.h

@@ -274,7 +274,8 @@ PositionWithAffinity PositionRespectingEditingBoundary(
 //  <editable>|<non-editable>abc</non-editable></editable>
 PositionWithAffinity AdjustForEditingBoundary(const PositionWithAffinity&);
 
-Position ComputePositionForNodeRemoval(const Position&, const Node&);
+CORE_EXPORT Position ComputePositionForNodeRemoval(const Position&,
+                                                   const Node&);
 
 // TODO(editing-dev): These two functions should be eliminated.
 CORE_EXPORT Position PositionBeforeNode(const Node&);

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

@@ -12,6 +12,23 @@ namespace blink {
 
 class EditingUtilitiesTest : public EditingTestBase {};
 
+TEST_F(EditingUtilitiesTest, ComputePositionForNodeRemovalAfterChildren) {
+  SetBodyContent("<div id=a><p id=b><img id=c></p></div>");
+  const Position position = Position::LastPositionInNode(*GetElementById("c"));
+  // Simulate <p> will be removed.
+  EXPECT_EQ(Position(*GetElementById("a"), 0),
+            ComputePositionForNodeRemoval(position, *GetElementById("b")));
+}
+
+TEST_F(EditingUtilitiesTest, ComputePositionForNodeRemovalAfterNode) {
+  // "editing/deleting/delete-start-block.html" hits this case.
+  SetBodyContent("<div id=a><p id=b><img id=c></p></div>");
+  const Position position = Position::AfterNode(*GetElementById("c"));
+  // Simulate <p> will be removed.
+  EXPECT_EQ(Position(*GetElementById("a"), 0),
+            ComputePositionForNodeRemoval(position, *GetElementById("b")));
+}
+
 TEST_F(EditingUtilitiesTest, DirectionOfEnclosingBlockOf) {
   const char* body_content =
       "<p id='host'><b id='one'></b><b id='two'>22</b></p>";