chromium: Add Gtk 3 for versions >= 56

Versions before 56 already had experimental support for Gtk 3 and since
version 56, Gtk 3 _seemed_ to become the default. Although it's now
requiring *both* Gtk 2 and Gtk3, so let's supply the dependency for now
to get it to build.

In the future however we might want to add use_gtk3 to the GN flags and
get rid of Gtk 2 completely.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>