Avoid -O3 in nacl as it does not play well with -W.

build/toolchain.gypi

 # Copyright 2013 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
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@@ skipping 543 matching lines @@
                 'cflags': [
                   '-fdata-sections',
                   '-ffunction-sections',
                   '-O1', # TODO(2807) should be -O3.
                 ],
               }],
               ['v8_optimized_debug==2', {
                 'cflags!': [
                   '-O0',
                   '-O1',
-                  '-O2',
                   '-Os',
                 ],
                 'cflags': [
                   '-fdata-sections',
                   '-ffunction-sections',
-                  '-O3',
                 ],
+                'conditions': [
+                  # TODO(crbug.com/272548): Avoid -O3 in NaCl
+                  ['nacl_target_arch=="none"', {
+                    'cflags': ['-O3'],
+                    'cflags!': ['-O2'],
+                    }, {
+                    'cflags': ['-O2'],
+                    'cflags!': ['-O3'],
+                  }],
+                ]
               }],
               ['v8_optimized_debug!=0 and gcc_version==44 and clang==0', {
                 'cflags': [
                   # Avoid crashes with gcc 4.4 in the v8 test suite.
                   '-fno-tree-vrp',
                 ],
               }],
             ],
           }],
           ['OS=="linux" and v8_enable_backtrace==1', {
@@ skipping 25 matching lines @@
                  }],
                ],
             },
           }],
         ],
       },  # Debug
       'Release': {
         'conditions': [
           ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="netbsd"', {
             'cflags!': [
-              '-O2',
               '-Os',
             ],
             'cflags': [
               '-fdata-sections',
               '-ffunction-sections',
-              '-O3',
               '<(wno_array_bounds)',
             ],
             'conditions': [
               [ 'gcc_version==44 and clang==0', {
                 'cflags': [
                   # Avoid crashes with gcc 4.4 in the v8 test suite.
                   '-fno-tree-vrp',
                 ],
               }],
-            ],
+              # TODO(crbug.com/272548): Avoid -O3 in NaCl
+              ['nacl_target_arch=="none"', {
+                'cflags': ['-O3'],
+                'cflags!': ['-O2'],
+              }, {
+                'cflags': ['-O2'],
+                'cflags!': ['-O3'],
+              }],
+            ]
           }],
           ['OS=="android"', {
             'cflags!': [
               '-O3',
               '-Os',
             ],
             'cflags': [
               '-fdata-sections',
               '-ffunction-sections',
               '-O2',
@@ skipping 43 matching lines @@
                 'OptimizeReferences': '2',
                 'EnableCOMDATFolding': '2',
               },
             },
           }],  # OS=="win"
         ],  # conditions
       },  # Release
     },  # configurations
   },  # target_defaults
 }