GN: Change webrtc_base -> rtc_base

The target was renamed in GYP a while back
and in https://code.google.com/p/webrtc/source/detail?r=8117
for GN. This is the last reference left using the
'webrtc_base' target name.

TBR=tommi@chromium.org

Review URL: https://codereview.chromium.org/868573003

Cr-Commit-Position: refs/heads/master@{#313100}

third_party/libjingle/BUILD.gn

@@ -260,7 +260,7 @@ static_library("libjingle") {
     ":jingle_deps",
   ]
   deps = [
-    "//third_party/webrtc/base:webrtc_base",
+    "//third_party/webrtc/base:rtc_base",
     ":libjingle_p2p_constants",
   ]