Chrome preserves inline styles of first character in the deleting/replacing
if first visible thing is text. Safari preserves inline style of the first
content of the range (i.e., styles of `<img>` etc is also respected).
For now, due to the market share and Safari's behavior is not obviously better
than Chrome, let's try to emulate the Chrome's behavior. However, the rules are
complicated and probably depend on how it implements the deletion. Therefore,
this patch makes Gecko only scan following text node if selection range starts
from end of a text node.
Depends on D164520
Differential Revision: https://phabricator.services.mozilla.com/D164521
2594ee7d9c