Bug 1524119 - Properly get VP9 benchmark value. r=bryce

And not the threshold which is always 150.

Differential Revision: https://phabricator.services.mozilla.com/D18159

--HG--
extra : moz-landing-system : lando

dom/media/Benchmark.cpp

@@ -67,7 +67,7 @@ uint32_t VP9Benchmark::MediaBenchmarkVp9Fps() {
   if (!ShouldRun()) {
     return 0;
   }
-  return StaticPrefs::MediaBenchmarkVp9Threshold();
+  return StaticPrefs::MediaBenchmarkVp9Fps();
 }
 
 // static