api/web/reports/{ReportId}/layouts/{LayoutId}
Updates an existing mail/export layout for a specific report.
This is a full replacement of the layout's own fields: 'Name', 'Subject' and
'SendCondition' are written exactly as supplied, so omitting 'SendCondition'
(or sending null) clears it and the mail is then sent on every execution.
To preserve a send condition, read the layout first and echo the value back.
'UseAsDefault' is the exception, because it is a property of the report rather
than of this layout - setting it true unsets every sibling layout. It is
therefore tri-state: true makes this layout the default, false clears the flag
on this layout only, and omitting it leaves the report's current default
assignment untouched.