When pressing `ArrowLeft` key at:
```
<p>abc</p><p><span contenteditable=false>def</span>{}<br></p>
```
, caret is moved into the non-editable text because
`nsCaret::GetCaretFrameForNodeOffset` looks for a preceding text node from
the `BRFrame`. This is required if the preceding text node ends with a
collapsible white-space but followed by a `<br>` because the text frame
should not contain the white-space rect and `BRFrame` frame should be next
to it, i.e., the white-space looks like a overflown content.
So, for rendering a caret, the method needs to return non-editable text frame
even in the case, but for considering new caret position in the DOM tree, it
should not return non-editable text frame.
Therefore, this patch adds a param to `nsCaret::GetCaretFrameForNodeOffset()`
which forcibly return editable content frame or not and makes
`Selection::GetPrimaryOrCaretFrameForNodeOffset` call it with the new option
because its callers are the handler of caret navigation.
Differential Revision: https://phabricator.services.mozilla.com/D196259
