Spinoff from https://bugzilla.mozilla.org/show_bug.cgi?id=1648500#c3.
In bug 1648500 I make a change so that when the root scrollframe determines whether or not it needs scrollbars, it does so by comparing the composition size to the content size. But in some specific cases with extension popups, the composition size isn't updated until after the reflow that does this scrollbar computation. This causes
browser_ext_browserAction_popup_resize.js to fail, and so to work around the failure I'm avoiding using the composition size when there's no MobileViewportManager on the presShell. But ideally we want to figure out a better solution and drop the MVM check from this code.