[FIX] web_editor: toolbar arrow position in iframe

Before this commit the position of the arrow was incorrect when the
editor is inside an iframe.

For the calculation of `arrowLeftPos` we need the difference between
the selection rectangle border and the toolbar's left position.
While `correctedSelectionRect.right` is relative to the current iframe's
borders, `left` is relative to the top document's window, as it has been
offset with `parentContextRect.left`. We need both values to be relative
to the same referential in order to make this calculation make sense.

task-3347942

Part-of: odoo/odoo#123568