When using autocorrect, we should use `insertReplacementText` according
to https://github.com/w3c/input-events/issues/152. So I would like to
add eContentCommandReplaceText command for this.
Also, this command has an option that is source string text. When
processing text subsitution, parent process doesn't know whether
target replaced text is modified. So I add this option for check.
Differential Revision: https://phabricator.services.mozilla.com/D213511