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}

webrtc/call/call_perf_tests.cc

@@ -290,7 +290,7 @@ void CallPerfTest::TestAudioVideoSync(FecMode fec,
   observer.PrintResults();
 
   // In quick test synchronization may not be achieved in time.
-  if (field_trial::FindFullName("WebRTC-QuickPerfTest") != "Enabled") {
+  if (field_trial::IsEnabled("WebRTC-QuickPerfTest")) {
     EXPECT_EQ(1, metrics::NumSamples("WebRTC.Video.AVSyncOffsetInMs"));
   }
 }