Fix dialog z-index issue

Summary:
Fixes T7081. History here:

• JX.Scrollbar made the page scroll weird when a dialog came up because it was half-frame and half-document.
• I made it fully frame-level.
• But this wasn't really right; a better fix is to make it fully document-level.

Test Plan:

• Weird scroll on opening dialog is still fixed.
• iOS Safari no longer puts the mask over the dialog.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7081

Differential Revision: https://secure.phabricator.com/D11559