Bug 1492764 - Set .left class to headerbar box element as Gtk+ does, r=jhorak

Gtk+ sets .left/.right classes to GtkBox element at GtkHeaderBar according to button placement (left/right).
We don't support that now so set .left at least to be compatible with themes - BlueMenta for instance.

Differential Revision: https://phabricator.services.mozilla.com/D6657

--HG--
extra : moz-landing-system : lando

widget/gtk/WidgetStyleCache.cpp

@@ -741,6 +741,9 @@ CreateHeaderBarButtons()
 
   GtkWidget *buttonBox = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, buttonSpacing);
   gtk_container_add(GTK_CONTAINER(GetWidget(MOZ_GTK_HEADER_BAR)), buttonBox);
+  // We support only LTR headerbar layout for now.
+  gtk_style_context_add_class(gtk_widget_get_style_context(buttonBox),
+                              GTK_STYLE_CLASS_LEFT);
 
   WidgetNodeType buttonLayout[TOOLBAR_BUTTONS];
   int activeButtons =