Test field trial group with startswith rather than equals.

BUG=webrtc:7266

Review-Url: https://codereview.webrtc.org/2717973005
Cr-Commit-Position: refs/heads/master@{#16915}
diff --git a/webrtc/test/rtp_rtcp_observer.h b/webrtc/test/rtp_rtcp_observer.h
index 4856de7..f3c95a6 100644
--- a/webrtc/test/rtp_rtcp_observer.h
+++ b/webrtc/test/rtp_rtcp_observer.h
@@ -43,7 +43,7 @@
   virtual ~RtpRtcpObserver() {}
 
   virtual bool Wait() {
-    if (field_trial::FindFullName("WebRTC-QuickPerfTest") == "Enabled") {
+    if (field_trial::IsEnabled("WebRTC-QuickPerfTest")) {
       observation_complete_.Wait(kShortTimeoutMs);
       return true;
     }