Bug 1939220 - part 3: Make TextFragmentData::BoundaryData not depend on the editing host r=m_kato

Its role is scanning the DOM.  Therefore, it can know which node is an editing
host during a scan.  Therefore, it does not require the editing host parameter.
To fix that, some utility methods of `HTMLEditUtils` needs to work with
non-editable nodes.  Therefore, this patch also updates `HTMLEditUtils`.

Finally, this gets rid of the `NS_ASSERTION`s in the constructor of
`TextFragmentData` because it'll be available with non-editable nodes too.
Although current all callers assume that it works as in an editing host.

Differential Revision: https://phabricator.services.mozilla.com/D232934

editor/libeditor/HTMLEditSubActionHandler.cpp

@@ -3362,13 +3362,12 @@ HTMLEditor::DeleteTextAndNormalizeSurroundingWhiteSpaces(
                 *newCaretPosition.ContainerAs<nsIContent>(),
                 HTMLEditUtils::ClosestEditableBlockElementOrInlineEditingHost,
                 BlockInlineCheck::UseComputedDisplayStyle)) {
-      Element* editingHost = ComputeEditingHost();
       // Try to put caret next to immediately after previous editable leaf.
       nsIContent* previousContent =
           HTMLEditUtils::GetPreviousLeafContentOrPreviousBlockElement(
-              newCaretPosition, *editableBlockElementOrInlineEditingHost,
-              {LeafNodeType::LeafNodeOrNonEditableNode},
-              BlockInlineCheck::UseComputedDisplayStyle, editingHost);
+              newCaretPosition, {LeafNodeType::LeafNodeOrNonEditableNode},
+              BlockInlineCheck::UseComputedDisplayStyle,
+              editableBlockElementOrInlineEditingHost);
       if (previousContent &&
           !HTMLEditUtils::IsBlockElement(
               *previousContent, BlockInlineCheck::UseComputedDisplayStyle)) {
@@ -3383,10 +3382,9 @@ HTMLEditor::DeleteTextAndNormalizeSurroundingWhiteSpaces(
       else if (nsIContent* nextContent =
                    HTMLEditUtils::GetNextLeafContentOrNextBlockElement(
                        newCaretPosition,
-                       *editableBlockElementOrInlineEditingHost,
                        {LeafNodeType::LeafNodeOrNonEditableNode},
                        BlockInlineCheck::UseComputedDisplayStyle,
-                       editingHost)) {
+                       editableBlockElementOrInlineEditingHost)) {
         newCaretPosition = nextContent->IsText() ||
                                    HTMLEditUtils::IsContainerNode(*nextContent)
                                ? EditorDOMPoint(nextContent, 0)