Line boxes don't contribute to scrollable overflow

[jgraham]

Description
In Gecko line boxes contribute directly to scrollable overflow, whereas in Chromium only the text within those line boxes contributes to scrollable overflow.

Bug reports
CHR-1082517
https://bugzilla.mozilla.org/show_bug.cgi?id=1637511
webcompat/web-bugs#53125

Specification
https://drafts.csswg.org/css-overflow-3/#scrollable

Tests
?

This is a bug to include in #9

[foolip]

In discussion with @bfgeek, the Chrome team can try to ship https://bugs.chromium.org/p/chromium/issues/detail?id=1082517 again, but if that breaks too much, can we revisit the spec'd behavior and try changing Gecko instead?

When testing https://bug1637511.bmoattachments.org/attachment.cgi?id=9148082 I see the same behavior in Chrome and Safari, and being able to scroll to the text does seem like it's better.

Also, having WebKit on board with trying to change this seems important. cc @jensimmons @gsnedders

[jensimmons]

Testing in Safari, it looks like WebKit matches the spec.

Safari produces scrollbars for both blocks:

[gsnedders]

When testing https://bug1637511.bmoattachments.org/attachment.cgi?id=9148082 I see the same behavior in Chrome and Safari, and being able to scroll to the text does seem like it's better.

I see Safari and Firefox matching?