Draw native GTK window nativation buttons on Linux

This CL is the initial implementation of GTK-rendered close, minimize,
and maximize buttons.  A demo can be seen at [1].  The implementation
is not ready to be released yet (still need to handle different scale
factors and button drawing for "backdrop" windows), so the feature is
guarded by a flag for now.

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=753067#c3

BUG=753067
R=sky@chromium.org,pkasting@chromium.org

Change-Id: Iadb5c96052f100097a10a5161aa3d9d44d89c901
Reviewed-on: https://chromium-review.googlesource.com/604288
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Elliot Glaysher <erg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496031}
29 files changed