Fix compilation error by unused functions under ceratin build configuration.

Existing code will not build when building chromeos_unittests target following the simple chrome build instruction
http://www.chromium.org/chromium-os/how-tos-and-troubleshooting/building-chromium-browser
with BOARD=x86-generic, for example.

TEST=chromeos_unittest built successfully following the instruction above, and the test passed.

Review-Url: https://codereview.chromium.org/2344893002
Cr-Commit-Position: refs/heads/master@{#419128}

chromeos/disks/disk_mount_manager_unittest.cc

@@ -391,24 +391,11 @@ 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() {}