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/sdk/objc/Framework/Classes/videotoolboxvideocodecfactory.cc

@@ -22,7 +22,7 @@ namespace {
 const char kHighProfileExperiment[] = "WebRTC-H264HighProfile";
 
 bool IsHighProfileEnabled() {
-  return field_trial::FindFullName(kHighProfileExperiment) == "Enabled";
+  return field_trial::IsEnabled(kHighProfileExperiment);
 }
 }