Web MIDI: add an unit test to check MidiManager instantiation

media/midi/midi_manager_usb_unittest.cc

Index: media/midi/midi_manager_usb_unittest.cc
diff --git a/media/midi/midi_manager_usb_unittest.cc b/media/midi/midi_manager_usb_unittest.cc
index b9a2732a81ec56aac64ebf94463fee4387c38a38..06ff33877d978d8223b8700865cf49526753a1ff 100644
--- a/media/midi/midi_manager_usb_unittest.cc
+++ b/media/midi/midi_manager_usb_unittest.cc
@@ -6,6 +6,7 @@
 
 #include <string>
 
+#include "base/message_loop/message_loop.h"
 #include "base/strings/stringprintf.h"
 #include "base/time/time.h"
 #include "media/midi/usb_midi_device.h"
@@ -141,7 +142,7 @@ class MidiManagerUsbForTesting : public MidiManagerUsb {
 
 class MidiManagerUsbTest : public ::testing::Test {
  public:
-  MidiManagerUsbTest() {
+  MidiManagerUsbTest() : message_loop_(new base::MessageLoop) {
     scoped_ptr<TestUsbMidiDeviceFactory> factory(new TestUsbMidiDeviceFactory);
     factory_ = factory.get();
     manager_.reset(
@@ -179,6 +180,8 @@ class MidiManagerUsbTest : public ::testing::Test {
   Logger logger_;
 
  private:
+  scoped_ptr<base::MessageLoop> message_loop_;
+
   DISALLOW_COPY_AND_ASSIGN(MidiManagerUsbTest);
 };