Update SerializeMediaStreamIds() signarure to use WTF::Vector

... instead of WebVector.

It avoids a needless copy/conversion.

BUG=787254
R=haraken@chromium.org

Change-Id: I16766e7906e3ba53a2fc43725f4555e55bee511f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1991409
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729700}

third_party/blink/renderer/modules/peerconnection/peer_connection_tracker.cc

@@ -117,7 +117,7 @@ String SerializeAnswerOptions(blink::RTCAnswerOptionsPlatform* options) {
   return result.ToString();
 }
 
-String SerializeMediaStreamIds(const blink::WebVector<String>& stream_ids) {
+String SerializeMediaStreamIds(const Vector<String>& stream_ids) {
   if (!stream_ids.size())
     return "[]";
   StringBuilder result;