Don't warn on missing prop synthesis in non-chromium code for iOS.

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 2476 matching lines @@
               }],
             ],
           }],
           [ 'OS=="ios"', {
             'xcode_settings': {
               # TODO(ios): Fix remaining warnings in third-party code, then
               # remove this; the Mac cleanup didn't get everything that's
               # flagged in an iOS build.
               'GCC_TREAT_WARNINGS_AS_ERRORS': 'NO',
               'RUN_CLANG_STATIC_ANALYZER': 'NO',
+              # Several internal ios directories generate numerous warnings for
+              # -Wobjc-missing-property-synthesis.
+              'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'NO',
             },
           }],
         ],
       }, {
         'includes': [
            # Rules for excluding e.g. foo_win.cc from the build on non-Windows.
           'filename_rules.gypi',
         ],
         # In Chromium code, we define __STDC_foo_MACROS in order to get the
         # C99 macros on Mac and Linux.
@@ skipping 2204 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }