Fix compilation error by unused functions under ceratin build configuration.

chromeos/disks/disk_mount_manager_unittest.cc

Index: chromeos/disks/disk_mount_manager_unittest.cc
diff --git a/chromeos/disks/disk_mount_manager_unittest.cc b/chromeos/disks/disk_mount_manager_unittest.cc
index 7b03b92daf4c73422853761ef7f431746dcea506..c20c523395d0cfa3cf855285ef4ee3ea74641ced 100644
--- a/chromeos/disks/disk_mount_manager_unittest.cc
+++ b/chromeos/disks/disk_mount_manager_unittest.cc
@@ -392,23 +392,10 @@ class MockDiskMountManagerObserver : public DiskMountManager::Observer {
 // Shift operators of ostream.
 // Needed to print values in case of EXPECT_* failure in gtest.
 std::ostream& operator<<(std::ostream& stream,
-                         const DeviceEvent& device_event) {
-  return stream << device_event.DebugString();
-}
-
-std::ostream& operator<<(std::ostream& stream, const DiskEvent& disk_event) {
-  return stream << disk_event.DebugString();
-}
-
-std::ostream& operator<<(std::ostream& stream,
                          const FormatEvent& format_event) {
   return stream << format_event.DebugString();
 }
 
-std::ostream& operator<<(std::ostream& stream, const MountEvent& mount_event) {
-  return stream << mount_event.DebugString();
-}
-
 class DiskMountManagerTest : public testing::Test {
  public:
   DiskMountManagerTest() {}