Update libjingle's GN build config to match a recent fix.

See here: https://codereview.chromium.org/516143002/

TBR=ajm

Review URL: https://codereview.chromium.org/523073003

Cr-Commit-Position: refs/heads/master@{#292792}

third_party/libjingle/BUILD.gn

@@ -19,6 +19,7 @@ config("jingle_unexported_configs") {
     "SRTP_RELATIVE_PATH",
     "USE_WEBRTC_DEV_BRANCH",
     "ENABLE_EXTERNAL_AUTH",
+    "WEBRTC_CHROMIUM_BUILD",
   ]
 
   include_dirs = [