[GTK3] Fix Chromium maximized maximize button

Summary: Chromium now uses the restore icon instead of the maximize icon when maximized.

Test Plan:
Before:
{F7877991}
After:
{F7877982}

Reviewers: #breeze, #vdg, ngraham

Reviewed By: #breeze, #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26592