GN: Refactor base/BUILD.gn and fix dbus-glib error.

Refactor webrtc/base/BUILD.gn to not have any subtracted
source entries.

Also fix an error in webrtc/BUILD.gn that occurs when running
on Chormium trybots as a part of enabling WebRTC for GN in
https://codereview.chromium.org/321313006/
The error is that pkg-config for dbus-glib fails. Workaround
this by putting the pkg-config entry within the proper condition.

BUG=webrtc:3441
TEST=
Successful compilation of WebRTC as standalone:
gn gen out/Default --args="build_with_chromium=false" && ninja -C out/Default
gn gen out/Default --args="build_with_chromium=false is_clang=true clang_use_chrome_plugins=false" && ninja -C out/Default

I built successfully from a Chromium checkout (with
https://codereview.chromium.org/321313006/ applied) using:
gn gen out/Default && ninja -C out/Default webrtc

R=brettw@chromium.org, henrike@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/16759004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6560 4adac7df-926f-26a2-2b94-8c16560cd09d