Update usage of GSettings in proxy config on linux

We were using a deprecated API to determine the existence of a schema,
and we were lazy loading the libgio library in case a distro doesn't
have the library.

All supported distros have libgio and support the new API, hence I fixed
both.

Test: Forced use of GSettings (see patch 1 - I use i3) and watched
chrome: //net-internals#proxy while I changed gsettings
Bug: 768027
Change-Id: Id9a3b157723c0b34e69b5b4eadcce1a7048b8636
Reviewed-on: https://chromium-review.googlesource.com/812471
Commit-Queue: Tim Brown <timbrown@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522686}
6 files changed