Use DownloadManager to initiate downloads from PluginInstaller.

chrome/chrome_tests_unit.gypi

Index: chrome/chrome_tests_unit.gypi
diff --git a/chrome/chrome_tests_unit.gypi b/chrome/chrome_tests_unit.gypi
index 66fa6d60f09968d72ca02725d8c15f6755f7fc7f..ad90d5b94e8458cc88a8913e071917fc01409e10 100644
--- a/chrome/chrome_tests_unit.gypi
+++ b/chrome/chrome_tests_unit.gypi
@@ -1132,6 +1132,7 @@
         'browser/password_manager/password_store_x_unittest.cc',
         'browser/performance_monitor/database_unittest.cc',
         'browser/plugins/plugin_finder_unittest.cc',
+        'browser/plugins/plugin_installer_unittest.cc',
         'browser/plugins/plugin_metadata_unittest.cc',
         'browser/plugins/plugin_prefs_unittest.cc',
         'browser/policy/cloud/cloud_policy_invalidator_unittest.cc',
@@ -2678,6 +2679,11 @@
             ['exclude', '^browser/chromeos/events/'],  # crbug.com/354036
           ],
         }],
+        ['enable_plugin_installation==0', {
+          'sources!': [
+            'browser/plugins/plugin_installer_unittest.cc',
+          ],
+        }],
       ],
     },
     {