Re-add the threading assertion on Android WebMidi implementation

media/midi/java/src/org/chromium/midi/MidiManagerAndroid.java

Index: media/midi/java/src/org/chromium/midi/MidiManagerAndroid.java
diff --git a/media/midi/java/src/org/chromium/midi/MidiManagerAndroid.java b/media/midi/java/src/org/chromium/midi/MidiManagerAndroid.java
index e657d17a818319e22e06b56ca76108cb6c243c40..e0d208d9d13ae3ce720c93f622f8592268b65c04 100644
--- a/media/midi/java/src/org/chromium/midi/MidiManagerAndroid.java
+++ b/media/midi/java/src/org/chromium/midi/MidiManagerAndroid.java
@@ -67,8 +67,7 @@ class MidiManagerAndroid {
      * @param nativeManagerPointer The native pointer to a midi::MidiManagerAndroid object.
      */
     private MidiManagerAndroid(Context context, long nativeManagerPointer) {
-        // TODO(crbug.com/665157)
-        // assert ThreadUtils.runningOnUiThread();
+        assert !ThreadUtils.runningOnUiThread();
 
         mManager = (MidiManager) context.getSystemService(Context.MIDI_SERVICE);
         mHandler = new Handler(ThreadUtils.getUiThreadLooper());