[Backport]Fixed Linux build with glib 2.40 (Ubuntu 14.04).

In proxy_config_service_linux, disabled glib deprecation warnings. The
function g_settings_list_schemas got deprecated in 2.40, but its
replacement is not available in older glib versions (such as the one the
build bots currently use, 2.32), so for now, disable the warnings.

BUG=351911
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=268417