Draw profile chooser button with GTK when possible

This is the initial implementation of drawing a native GTK button as
the profile chooser button on Linux.  There are still some tasks left
(like fixing HIDPI rendering), so the feature is guarded behind an
--enable-native-avatar-button flag.

Demo:
https://bugs.chromium.org/p/chromium/issues/detail?id=753067#c46

BUG=753067

Change-Id: Iff57d293a04bf2259aa0bb77161b51fec6370289
Reviewed-on: https://chromium-review.googlesource.com/703482
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513338}
37 files changed