Gtk3: Fix frame button scaling on HIDPI

This CL fixes frame buttons being too small when GDK_SCALE=2, as seen
in [1].

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=753067#c43

BUG=753067
R=erg@chromium.org

Change-Id: I835668b779ab10ab3da971960794093f37813141
Reviewed-on: https://chromium-review.googlesource.com/729568
Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510498}
1 file changed