linux_aura: Default to system titlebars under some window managers.

ui/base/x/x11_util.h

Index: ui/base/x/x11_util.h
diff --git a/ui/base/x/x11_util.h b/ui/base/x/x11_util.h
index 92ad3f0e2512495ee0eb4fde925ec241a8d8975f..dbcd987b9dd2984ac7b746302477c332c1602aeb 100644
--- a/ui/base/x/x11_util.h
+++ b/ui/base/x/x11_util.h
@@ -224,6 +224,10 @@ UI_BASE_EXPORT XID GetParentWindow(XID window);
 // Walk up |window|'s hierarchy until we find a direct child of |root|.
 XID GetHighestAncestorWindow(XID window, XID root);
 
+// Determine whether we should default to native decorations or the custom
+// frame based on the currently-running window manager.
+UI_BASE_EXPORT bool GetCustomFramePrefDefault();
+
 static const int kAllDesktops = -1;
 // Queries the desktop |window| is on, kAllDesktops if sticky. Returns false if
 // property not found.