Make |use_glib| configurable.

This is needed for mojo. We're trying to remove glib and dbus from our build, and need to be able to configure this option. This will be rolled back into the mojo repository.

This was split from the mojo patch: https://codereview.chromium.org/853133002/

BUG=443439

Review URL: https://codereview.chromium.org/853103004

Cr-Original-Commit-Position: refs/heads/master@{#312261}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9252aff86f89f9ae0293a81b46097b1b51a6b553

config/ui.gni

@@ -29,13 +29,16 @@ declare_args() {
 
   # True means the UI is built using the "views" framework.
   toolkit_views = is_win || is_chromeos || use_aura
+
+  # Whether we should use glib, a low level C utility library.
+  use_glib = is_linux && !use_ozone
 }
 
 # Additional dependent variables -----------------------------------------------
 #
 # These variables depend on other variables and can't be set externally.
 
-if (is_linux) {
+if (is_linux && use_glib) {
   use_cairo = true
   use_pango = true
 } else {
@@ -54,8 +57,6 @@ use_x11 = is_linux && !use_ozone
 
 use_ozone_evdev = use_ozone
 
-use_glib = is_linux && !use_ozone
-
 use_clipboard_aurax11 = is_linux && use_aura && use_x11
 
 use_athena = false