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/media/engine/internalencoderfactory.cc b/webrtc/media/engine/internalencoderfactory.cc
index ad3a4be..65c1ba9 100644
--- a/webrtc/media/engine/internalencoderfactory.cc
+++ b/webrtc/media/engine/internalencoderfactory.cc
@@ -24,7 +24,7 @@
 const char kFlexfecFieldTrialName[] = "WebRTC-FlexFEC-03";
 
 bool IsFlexfecFieldTrialEnabled() {
-  return webrtc::field_trial::FindFullName(kFlexfecFieldTrialName) == "Enabled";
+  return webrtc::field_trial::IsEnabled(kFlexfecFieldTrialName);
 }
 
 }  // namespace