Fix prerender histograms for multiple prerender case.

chrome/chrome_tests.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'variables' : {
     'pyautolib_sources': [
       'app/chrome_command_ids.h',
       'app/chrome_dll_resource.h',
       'common/automation_constants.h',
       'common/pref_names.cc',
@@ skipping 1514 matching lines @@
         'browser/prefs/pref_service_unittest.cc',
         'browser/prefs/pref_set_observer_unittest.cc',
         'browser/prefs/pref_value_map_unittest.cc',
         'browser/prefs/pref_value_store_unittest.cc',
         'browser/prefs/proxy_config_dictionary_unittest.cc',
         'browser/prefs/proxy_policy_unittest.cc',
         'browser/prefs/proxy_prefs_unittest.cc',
         'browser/prefs/scoped_user_pref_update_unittest.cc',
         'browser/prefs/session_startup_pref_unittest.cc',
         'browser/prerender/prerender_history_unittest.cc',
+        'browser/prerender/prerender_manager_unittest.cc',
         'browser/prerender/prerender_tracker_unittest.cc',
         'browser/prerender/prerender_unittest.cc',
         'browser/prerender/prerender_util_unittest.cc',
         'browser/printing/cloud_print/cloud_print_proxy_service_unittest.cc',
         'browser/printing/cloud_print/cloud_print_setup_source_unittest.cc',
         'browser/printing/print_dialog_cloud_unittest.cc',
         'browser/printing/print_job_unittest.cc',
         'browser/printing/print_preview_tab_controller_unittest.cc',
         'browser/process_info_snapshot_mac_unittest.cc',
         'browser/process_singleton_linux_unittest.cc',
@@ skipping 3362 matching lines @@
                 '--result', '<@(_outputs)',
                 '--isolate', 'browser_tests.isolate',
               ],
             },
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }