Add -Wobjc-missing-property-synthesis to comment on symbolic name

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 3110 matching lines @@
             # development except for ASan builds.
             ['clang==1', {
               'CC': '$(SOURCE_ROOT)/<(clang_dir)/clang',
               'LDPLUSPLUS': '$(SOURCE_ROOT)/<(clang_dir)/clang++',
 
               # Don't use -Wc++0x-extensions, which Xcode 4 enables by default
               # when building with clang. This warning is triggered when the
               # override keyword is used via the OVERRIDE macro from
               # base/compiler_specific.h.
               'CLANG_WARN_CXX0X_EXTENSIONS': 'NO',
-              # Warn if automatic synthesis is triggered.
+              # Warn if automatic synthesis is triggered with
+              # the -Wobjc-missing-property-synthesis flag.
               'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'YES',
               'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
               'WARNING_CFLAGS': [
                 '-Wheader-hygiene',
                 # Don't die on dtoa code that uses a char as an array index.
                 # This is required solely for base/third_party/dmg_fp/dtoa.cc.
                 '-Wno-char-subscripts',
                 # Clang spots more unused functions.
                 '-Wno-unused-function',
                 # See comments on this flag higher up in this file.
@@ skipping 209 matching lines @@
 
           # This next block is mostly common with the 'mac' section above,
           # but keying off (or setting) 'clang' isn't valid for iOS as it
           # also seems to mean using the custom build of clang.
 
           # Don't use -Wc++0x-extensions, which Xcode 4 enables by default
           # when building with clang. This warning is triggered when the
           # override keyword is used via the OVERRIDE macro from
           # base/compiler_specific.h.
           'CLANG_WARN_CXX0X_EXTENSIONS': 'NO',
-          # Warn if automatic synthesis is triggered.
+          # Warn if automatic synthesis is triggered with
+          # the -Wobjc-missing-property-synthesis flag.
           'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'YES',
           'WARNING_CFLAGS': [
             '-Wheader-hygiene',
             # Don't die on dtoa code that uses a char as an array index.
             # This is required solely for base/third_party/dmg_fp/dtoa.cc.
             '-Wno-char-subscripts',
             # Clang spots more unused functions.
             '-Wno-unused-function',
             # See comments on this flag higher up in this file.
             '-Wno-unnamed-type-template-args',
@@ skipping 402 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',
   },
 }