Make IsEditablePosition() to use Position::ComputeContainerNode()

This patch changes |IsEditablePosition()| to use |Position::
ComputeContainerNode()| instead of |Position::ParentAnchoredEquivalent()| not to
incorporate |EditorIngoreContetns()|, e.g. <hr>, <img>, etc.

For "<hr contenteditable>", |IsEditablePosition(<hr>@0)| returns false before
this patch. After this patch, it returns true.

Bug: 873088
Change-Id: Id75bd2c857dbd85e59f7795d02f3583802347060
Reviewed-on: https://chromium-review.googlesource.com/1172273Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583457}

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

@@ -458,7 +458,7 @@ ContainerNode* HighestEditableRoot(const PositionInFlatTree& position) {
 }
 
 bool IsEditablePosition(const Position& position) {
-  const Node* node = position.ParentAnchoredEquivalent().AnchorNode();
+  const Node* node = position.ComputeContainerNode();
   if (!node)
     return false;
   DCHECK(node->GetDocument().IsActive());

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

@@ -104,6 +104,22 @@ TEST_F(EditingUtilitiesTest, enclosingNodeOfType) {
             EnclosingNodeOfType(PositionInFlatTree(one, 0), IsEnclosingBlock));
 }
 
+// http://crbug.com/873088
+TEST_F(EditingUtilitiesTest, IsEditablePositionWithHr) {
+  SetBodyContent("<hr contenteditable id=target>");
+  Element& target = *GetDocument().getElementById("target");
+  EXPECT_FALSE(IsEditablePosition(Position::BeforeNode(target)));
+  EXPECT_TRUE(IsEditablePosition(Position(target, 0)));
+}
+
+// http://crbug.com/873088
+TEST_F(EditingUtilitiesTest, IsEditablePositionWithSpan) {
+  SetBodyContent("<span contenteditable id=target>abc</span>");
+  Element& target = *GetDocument().getElementById("target");
+  EXPECT_FALSE(IsEditablePosition(Position::BeforeNode(target)));
+  EXPECT_TRUE(IsEditablePosition(Position(target, 0)));
+}
+
 TEST_F(EditingUtilitiesTest, isEditablePositionWithTable) {
   // We would like to have below DOM tree without HTML, HEAD and BODY element.
   //   <table id=table><caption>foo</caption></table>