Take 3 at turning on -Werror. This passed the last run on the

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 129 matching lines @@
     },
   },
   'conditions': [
     [ 'OS=="linux"', {
       'target_defaults': {
         'asflags': [
           # Needed so that libs with .s files (e.g. libicudata.a)
           # are compatible with the general 32-bit-ness.
           '-32',
         ],
+        # Enable -Werror by default, but put it in a variable so it can
+        # be disabled in ~/.gyp/include.gypi on the valgrind builders.
+        'variables': {
+          'werror%': '-Werror',
+        },
         # All floating-point computations on x87 happens in 80-bit
         # precision.  Because the C and C++ language standards allow
         # the compiler to keep the floating-point values in higher
         # precision than what's specified in the source and doing so
         # is more efficient than constantly rounding up to 64-bit or
         # 32-bit precision as specified in the source, the compiler,
         # especially in the optimized mode, tries very hard to keep
         # values in x87 floating-point stack (in 80-bit precision)
         # as long as possible. This has important side effects, that
         # the real value used in computation may change depending on
@@ skipping 13 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)',  # See note above about the werror variable.
         ],
         '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 56 matching lines @@
             'defines': [
               '_DEBUG',
             ],
             'cflags': [
               '-O<(debug_optimize)',
               '-g',
             ],
             'ldflags': [
               '-rdynamic',  # Allows backtrace to resolve symbols.
             ],
-          },
+          },
           'Release': {
             'cflags': [
               '-O2',
               # Don't emit the GCC version ident directives, they just end up
               # in the .comment section taking up binary size.
               '-fno-ident',
               # Put data and code in their own sections, so that unused symbols
               # can be removed at link time with --gc-sections.
               '-fdata-sections',
               '-ffunction-sections',
@@ skipping 157 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',
   },
 }