Support arbitrary scale factors on GTK-rendered frame buttons

Please see the demo:
https://bugs.chromium.org/p/chromium/issues/detail?id=753067#c7

Note that the Chrome scale behavior differs slightly from the GTK
behavior: GTK does not support fractional scale factors, but Chrome
does.  As such, GTK never renders a frame button with eg. 1.5x scale.
However, since all parts of the Chrome UI (including the tabstrip
background) scale this way, we render the frame buttons at 1.5x scale
anyway.

BUG=753067
R=erg@chromium.org

Change-Id: Ief59d3626878d7a7da6f01b7aac2ca8dac802ab2
Reviewed-on: https://chromium-review.googlesource.com/625240
Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496350}
1 file changed