Reland of GN: Add BUILD.gn files for webrtc/{api,media,libjingle,p2p,pc}

webrtc/pc/pc.gyp

 # Copyright (c) 2016 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.
 
 {
   'includes': ['../build/common.gypi'],
   'variables': {
     'rtc_pc_defines': [
       'SRTP_RELATIVE_PATH',
       'HAVE_SCTP',
       'HAVE_SRTP',
     ],
   },
   'targets': [
     {
       'target_name': 'rtc_pc',
       'type': 'static_library',
       'dependencies': [
         '<(webrtc_root)/base/base.gyp:rtc_base',
         '<(webrtc_root)/media/media.gyp:rtc_media',
       ],
       'conditions': [
+        ['build_with_chromium==1', {
+          'sources': [
+            'externalhmac.h',
+            'externalhmac.cc',
+          ],
+        }],
         ['build_libsrtp==1', {
           'dependencies': [
             '<(DEPTH)/third_party/libsrtp/libsrtp.gyp:libsrtp',
           ],
         }],
       ],
       'defines': [
         '<@(rtc_pc_defines)',
       ],
       'include_dirs': [
@@ skipping 94 matching lines @@
               'sources': [
                 'rtc_pc_unittests.isolate',
               ],
             },
           ],
         }],
       ],  # conditions
     }],  # include_tests==1
   ],  # conditions
 }