Bug 1677635 Part 4 - Rename nsOverflowAreas to OverflowAreas, and move it into mozilla namespace. r=layout-reviewers,emilio

This patch is generated via the rename functionality in my editor; add
`mozilla::` prefix to `OverflowAreas` in headers; and remove the
`OverflowType` alias added in Part 1.

Differential Revision: https://phabricator.services.mozilla.com/D97235