Web MIDI dynamic instantiation experiment-controlled rollouts: step 4

Enable dynamic instantiation mode by default on Linux, ChromeOS,
and Windows. Other platforms still need to rely on the existing
experiment-controlled flag and will be launched separately.

BUG=672793,714511

Review-Url: https://codereview.chromium.org/2835083002
Cr-Commit-Position: refs/heads/master@{#466881}

media/midi/midi_service.cc

@@ -17,10 +17,10 @@ namespace {
 bool IsDynamicInstantiationEnabled() {
 // TODO(toyoshim): Support on all platforms. See https://crbug.com/672793.
 #if defined(OS_LINUX) || defined(OS_WIN)
+  return true;
+#else
   return base::FeatureList::IsEnabled(
       features::kMidiManagerDynamicInstantiation);
-#else
-  return false;
 #endif
 }