Add meta viewport tag for WPT test scrollable-overflow-padding.html.
Without this change, the test's viewport gets automatically zoomed out when
run on Android; and that zoom factor can lead to off-by-one test failures from
pixel-rounding discrepancies.
Differential Revision: https://phabricator.services.mozilla.com/D229023
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1930380
gecko-commit: f9c3d214198869ecaa968022339f9b1a210b5c52
gecko-reviewers: dholbert