Revert of Remove {media,p2p,pc,xmllite,xmpp}_tests.gypi files.

webrtc/pc/rtc_pc_unittests.isolate

-# Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
-#
-# Use of this source code is governed by a BSD-style license
-# that can be found in the LICENSE file in the root of the source
-# tree. An additional intellectual property rights grant can be found
-# in the file PATENTS.  All contributing project authors may
-# be found in the AUTHORS file in the root of the source tree.
-
-{
-  'conditions': [
-    ['OS=="linux" or OS=="mac" or OS=="win"', {
-      'variables': {
-        'command': [
-          '<(PRODUCT_DIR)/rtc_pc_unittests<(EXECUTABLE_SUFFIX)',
-        ],
-        'files': [
-          '<(PRODUCT_DIR)/rtc_pc_unittests<(EXECUTABLE_SUFFIX)',
-        ],
-      },
-    }],
-  ],
-}