Refactor how clang warning flags are set.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index e1c5a3f4ab1300f90b9fd895930ffb09d2fb87e2..4f83f4c400bcc5bcb3ceeea20c7b4ed7f18bb061 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -2400,7 +2400,40 @@
           'host_os%': '<(host_os)',  # See comment above chromium_code.
         }],
       ],
+      'clang_warning_flags': [
+        '-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',
+
+        # TODO(thakis): This used to be implied by -Wno-unused-function,
+        # which we no longer use. Check if it makes sense to remove
+        # this as well. http://crbug.com/316352
+        '-Wno-unneeded-internal-declaration',
+
+        # Warns on switches on enums that cover all enum values but
+        # also contain a default: branch. Chrome is full of that.
+        '-Wno-covered-switch-default',
+
+        # Warns when a const char[] is converted to bool.
+        '-Wstring-conversion',
+
+        # C++11-related flags:
+
+        # This warns on using ints as initializers for floats in
+        # initializer lists (e.g. |int a = f(); CGSize s = { a, a };|),
+        # which happens in several places in chrome code. Not sure if
+        # this is worth fixing.
+        '-Wno-c++11-narrowing',
+
+        # Clang considers the `register` keyword as deprecated, but e.g.
+        # code generated by flex (used in angle) contains that keyword.
+        # http://crbug.com/255186
+        '-Wno-deprecated-register',
+      ],
     },
+    'includes': [ 'set_clang_warning_flags.gypi', ],
     'defines': [
       # Don't use deprecated V8 APIs anywhere.
       'V8_DEPRECATION_WARNINGS',
@@ -2873,18 +2906,7 @@
         'defines': ['OS_CHROMEOS=1'],
       }],
       ['enable_wexit_time_destructors==1', {
-        'conditions': [
-          [ 'clang==1', {
-            'cflags': [
-              '-Wexit-time-destructors',
-            ],
-            'xcode_settings': {
-              'WARNING_CFLAGS': [
-                '-Wexit-time-destructors',
-              ],
-            },
-          }],
-        ],
+        'variables': { 'clang_warning_flags': ['-Wexit-time-destructors']},
       }],
       ['chromium_code==0', {
         'conditions': [
@@ -3865,38 +3887,8 @@
           }],
           ['clang==1', {
             'cflags': [
-              '-Wheader-hygiene',
-
-              # Don't die on dtoa code that uses a char as an array index.
-              '-Wno-char-subscripts',
-
-              # TODO(thakis): This used to be implied by -Wno-unused-function,
-              # which we no longer use. Check if it makes sense to remove
-              # this as well. http://crbug.com/316352
-              '-Wno-unneeded-internal-declaration',
-
-              # Warns on switches on enums that cover all enum values but
-              # also contain a default: branch. Chrome is full of that.
-              '-Wno-covered-switch-default',
-
-              # Warns when a const char[] is converted to bool.
-              '-Wstring-conversion',
-
-              # C++11-related flags:
-
-              # This warns on using ints as initializers for floats in
-              # initializer lists (e.g. |int a = f(); CGSize s = { a, a };|),
-              # which happens in several places in chrome code. Not sure if
-              # this is worth fixing.
-              '-Wno-c++11-narrowing',
-
               # TODO(thakis): Remove, http://crbug.com/263960
               '-Wno-reserved-user-defined-literal',
-
-              # Clang considers the `register` keyword as deprecated, but e.g.
-              # code generated by flex (used in angle) contains that keyword.
-              # http://crbug.com/255186
-              '-Wno-deprecated-register',
             ],
             'cflags_cc': [
               # See the comment in the Mac section for what it takes to move
@@ -4675,35 +4667,6 @@
               'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'YES',
               'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
               'WARNING_CFLAGS': [
-                '-Wheader-hygiene',
-
-                # This warns on using ints as initializers for floats in
-                # initializer lists (e.g. |int a = f(); CGSize s = { a, a };|),
-                # which happens in several places in chrome code. Not sure if
-                # this is worth fixing.
-                '-Wno-c++11-narrowing',
-
-                # 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',
-
-                # TODO(thakis): This used to be implied by -Wno-unused-function,
-                # which we no longer use. Check if it makes sense to remove
-                # this as well. http://crbug.com/316352
-                '-Wno-unneeded-internal-declaration',
-
-                # Warns on switches on enums that cover all enum values but
-                # also contain a default: branch. Chrome is full of that.
-                '-Wno-covered-switch-default',
-
-                # Warns when a const char[] is converted to bool.
-                '-Wstring-conversion',
-
-                # Clang considers the `register` keyword as deprecated, but
-                # e.g. code generated by flex (used in angle) contains that
-                # keyword. http://crbug.com/255186
-                '-Wno-deprecated-register',
-
                 # This warns on selectors from Cocoa headers (-length, -set).
                 # cfe-dev is currently discussing the merits of this warning.
                 # TODO(thakis): Reevaluate what to do with this, based one