GN: Prefix WebRTC specific variables with "rtc_"

BUG=3441
TESTED=Trybots + Running GN in a Chromium checkout with
src/third_party/webrtc symlinked to the WebRTC checkout
with this CL applied, both with the default GN settings
and using: --args="os=\"android\" cpu_arch=\"arm\""

R=brettw@chromium.org

Review URL: https://webrtc-codereview.appspot.com/27379004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7095 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/video_coding/BUILD.gn

@@ -137,7 +137,7 @@ source_set("webrtc_vp8") {
     "../../common_video",
     "../../system_wrappers",
   ]
-#  if (build_libvpx) {
+#  if (rtc_build_libvpx) {
 #    deps += [
 #      "//third_party/libvpx",
 #    ]