Fix a bug that unmounting devices on the policy update can be a busy loop.

chrome/browser/chromeos/file_manager/volume_manager_unittest.cc

Index: chrome/browser/chromeos/file_manager/volume_manager_unittest.cc
diff --git a/chrome/browser/chromeos/file_manager/volume_manager_unittest.cc b/chrome/browser/chromeos/file_manager/volume_manager_unittest.cc
index 12eaa24fa0a7256133564b06d1c1e9113c33cad4..dee9b6eb5be0b62c2f7c1e246ece680e20cd36c2 100644
--- a/chrome/browser/chromeos/file_manager/volume_manager_unittest.cc
+++ b/chrome/browser/chromeos/file_manager/volume_manager_unittest.cc
@@ -722,6 +722,10 @@ TEST_F(VolumeManagerTest, OnExternalStorageDisabledChanged) {
   profile()->GetPrefs()->SetBoolean(prefs::kExternalStorageDisabled, true);
   volume_manager()->OnExternalStorageDisabledChanged();
 
+  // Wait until all unmount request finishes, so that callback chain to unmount
+  // all the mount points will be invoked.
+  disk_mount_manager_->FinishAllUnmountPathRequests();
+
   // The all mount points should be unmounted.
   EXPECT_EQ(0U, disk_mount_manager_->mount_points().size());