[GTK3/Firefox] Fix scrollbar click region

Summary:
Scrollbars now have a click region equivalent to their visual space in Firefox.

FIXED-IN: 5.17.3
BUG: 413118

Test Plan:
Before: 5px edge on scrollbars doesn't respond to clicks.

After: 5px edge on scrollbars responds to clicks.

Reviewers: #vdg, #breeze, ngraham

Reviewed By: #vdg, #breeze, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25180