Set the initial visible area with MobileViewportManager.
On mobile environments, specifically web contents without meta viewport tags get
rendered on 980px width device (i.e. desktop mode), thus using the document
viewer size directly for the pres context's visible area would result a resize event.
Using MobileViewportManager avoids the resize event.
Differential Revision: https://phabricator.services.mozilla.com/D220976
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1768527
gecko-commit: cca9937c0323ea91455a7bd6d72a311c89a4c475
gecko-reviewers: emilio, botond