message: Add ios_code_signing_identity_description to GN args.
  TBR=courtneyfe@webrtc.org
  Cr-Commit-Position: refs/heads/master@{#31335}
diff: 'ios', 'release_bot', 'arm64', 'no_ios_code_signing', 'ios_use_goma_rbe'
  ],
  'ios_internal_debug_bot_arm64': [
  - 'ios', 'debug_bot', 'arm64', 'ios_use_goma_rbe'
  + 'ios', 'debug_bot', 'arm64', 'ios_use_goma_rbe',
  + 'ios_code_signing_identity_description',
  ],
  'ios_internal_release_bot_arm64': [
  - 'ios', 'release_bot', 'arm64', 'ios_use_goma_rbe'
  + 'ios', 'release_bot', 'arm64', 'ios_use_goma_rbe',
  + 'ios_code_signing_identity_description',
  ],
  'ios_internal_pure_release_bot_arm64': [
  - 'ios', 'pure_release_bot', 'arm64', 'ios_use_goma_rbe'
  + 'ios', 'pure_release_bot', 'arm64', 'ios_use_goma_rbe',
  + 'ios_code_signing_identity_description',
  ],
  'ios_debug_bot_x64': [
  'ios', 'debug_bot', 'x64', 'ios_use_goma_rbe'
  'gn_args': 'use_goma=true',
  },
  + 'ios_code_signing_identity_description': {
  + 'gn_args': 'ios_code_signing_identity_description="Apple Development"',
  + },
  +
  'ios_use_goma_rbe': {
  'gn_args': 'ios_use_goma_rbe=true',
  },
  