When testing, we don't want to know about 'component' or 'external'...

chrome/test/automation/automation_messages_internal.h

 // Copyright (c) 2006-2008 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.
 
 // Defines the IPC messages used by the automation interface.
 
 // This header is meant to be included in multiple passes, hence no traditional
 // header guard.
 // See ipc_message_macros.h for explanation of the macros and passes.
 
@@ skipping 1143 matching lines @@
   IPC_SYNC_MESSAGE_ROUTED1_1(AutomationMsg_InstallExtension,
                              FilePath /* full path to crx file */,
                              AutomationMsg_ExtensionResponseValues)
 
   // Silently load the extension in the given directory.  This expects an
   // extension expanded into the directory, not a crx file.
   IPC_SYNC_MESSAGE_ROUTED1_1(AutomationMsg_LoadExpandedExtension,
                              FilePath /* root directory of extension */,
                              AutomationMsg_ExtensionResponseValues)
 
-  // Retrieves a list of the root directories of all enabled extensions.
+  // Retrieves a list of the root directories of all enabled extensions
+  // that have been installed into Chrome by dropping a .crx file onto
+  // Chrome or an equivalent action.  Other types of extensions are not
+  // included on the list (e.g. "component" extensions, "external"
+  // extensions or extensions loaded via --load-extension since the first
+  // two are generally not useful for testing (e.g. an external extension
+  // could mess with an automated test if it's present on some systems only)
+  // and the last would generally be explicitly loaded by tests.
   IPC_SYNC_MESSAGE_ROUTED0_1(AutomationMsg_GetEnabledExtensions,
                              std::vector<FilePath>)
 
   // This message requests the type of the window with the given handle. The
   // return value contains the type (Browser::Type), or -1 if the request
   // failed.
   IPC_SYNC_MESSAGE_ROUTED1_1(AutomationMsg_Type, int, int)
 
   // Opens a new browser window of a specific type.
   IPC_SYNC_MESSAGE_ROUTED2_0(AutomationMsg_OpenNewBrowserWindowOfType,
@@ skipping 100 matching lines @@
                              int /* browser_handle */,
                              int64 /* id */,
                              GURL /* url */,
                              bool /* success */)
   IPC_SYNC_MESSAGE_ROUTED2_1(AutomationMsg_RemoveBookmark,
                              int /* browser_handle */,
                              int64 /* id */,
                              bool /* success */)
 
 IPC_END_MESSAGES(Automation)