Use RunLoop, but do not call RunUntilIdle() multiple times

In unit tests for MidiManager, RunLoop was used, and is used to wait
a posted task finished. But, RunUntilIdle() can not be called
multiple times. To avoid this restriction, we can just create new
instance again for another iteration.

TEST=media_unittests

Review URL: https://codereview.chromium.org/656793004

Cr-Commit-Position: refs/heads/master@{#300240}

media/midi/midi_manager_unittest.cc

@@ -75,24 +75,11 @@ class FakeMidiManagerClient : public MidiManagerClient {
   int client_id() const { return client_id_; }
   MidiResult result() const { return result_; }
 
-  void HandleContinuationMessage() {
-    // Stop posting a dummy message once CompleteStartSession() is invoked.
-    if (!wait_for_result_)
-      return;
-    base::MessageLoop::current()->PostTask(
-        FROM_HERE,
-        base::Bind(&FakeMidiManagerClient::HandleContinuationMessage,
-                   base::Unretained(this)));
-  }
-
-
   MidiResult WaitForResult() {
-    base::RunLoop run_loop;
-    // Post a dummy task not to stop the following event loop.
-    HandleContinuationMessage();
-    // CompleteStartSession() is called inside the message loop on the same
-    // thread. Protection for |wait_for_result_| is not needed.
-    run_loop.RunUntilIdle();
+    while (wait_for_result_) {
+      base::RunLoop run_loop;
+      run_loop.RunUntilIdle();
+    }
     return result();
   }
 

media/midi/midi_manager_usb_unittest.cc

@@ -178,9 +178,10 @@ class MidiManagerUsbTest : public ::testing::Test {
   void RunCallbackUntilCallbackInvoked(
       bool result, UsbMidiDevice::Devices* devices) {
     factory_->callback_.Run(result, devices);
-    base::RunLoop run_loop;
-    while (!client_->complete_start_session_)
+    while (!client_->complete_start_session_) {
+      base::RunLoop run_loop;
       run_loop.RunUntilIdle();
+    }
   }
 
   scoped_ptr<MidiManagerUsbForTesting> manager_;