Take 2 at turning on -Werror. I think this will work on gcc 4.2,

build/common.gypi

 # Copyright (c) 2009 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.
 
 {
   'variables': {
     # .gyp files should set chromium_code to 1 if they build Chromium-specific
     # code, as opposed to external code.  This variable is used to control
     # such things as the set of warnings to enable, and whether warnings are
     # treated as errors.
@@ skipping 162 matching lines @@
         # compiler optimized the code, since the value is always kept
         # in its specified precision.
         'cflags': [
           '-m32',
           '-pthread',
           '-march=pentium4',
           '-fno-exceptions',
           '-msse2',
           '-mfpmath=sse',
           '-Wall',
-          # TODO(sgk): when chromium_code targets are clean of warnings.
-          #'-Werror',
+          '-Werror',
         ],
         'ldflags': [
           '-m32',
           '-pthread',
         ],
         'scons_variable_settings': {
           'LIBPATH': ['$LIB_DIR'],
           # Linking of large files uses lots of RAM, so serialize links
           # using the handy flock command from util-linux.
           'FLOCK_LINK': ['flock', '$TOP_BUILDDIR/linker.lock', '$LINK'],
@@ skipping 234 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',
   },
 }