Gtk: Explicitly track if nav buttons are set in GtkUi

GtkUi used to implicitly track if the nav button layout was set by
checking if the button layout was nonempty.  This caused a bug where
the button layout would incorrectly display the default button layout
(minimize, maximize, and close all on the right) when Chrome was
launched and the GTK setting had no frame buttons.

Note that it was still possible to get Chrome to display no frame
buttons if you changed the GTK setting after Chrome was launched.

BUG=21438
R=erg@chromium.org

Change-Id: Ie9f9f4d72b2009b600f50eda4c647dc20e2d5196
Reviewed-on: https://chromium-review.googlesource.com/633837
Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497456}
2 files changed