[GTK3] Fix Chromium maximized maximize button
Summary: Chromium now uses the restore icon instead of the maximize icon when maximized.
Test Plan:
Before:
After:
Reviewers: Breeze, VDG, ngraham
Reviewed By: Breeze, VDG, ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26592