Refactor how clang warning flags are set.

skia/skia_chrome.gypi

Index: skia/skia_chrome.gypi
diff --git a/skia/skia_chrome.gypi b/skia/skia_chrome.gypi
index 99c7134b233068179c8874d65bd59ff13d5a969a..1637e88c12c463ec89ead2439b62b46eb67def1c 100644
--- a/skia/skia_chrome.gypi
+++ b/skia/skia_chrome.gypi
@@ -19,7 +19,13 @@
       'ext',
     ],
   },
-
+  'variables': {
+    # TODO(scottmg): http://crbug.com/177306
+    'clang_warning_flags_unset': [
+      # Don't warn about string->bool used in asserts.
+      '-Wstring-conversion',
+    ],
+  },
   'sources': [
     # Note: file list duplicated in GN build.
     'ext/analysis_canvas.cc',
@@ -99,18 +105,6 @@
         'skia_chrome_opts',
       ],
     }],
-    # TODO(scottmg): http://crbug.com/177306
-    ['clang==1', {
-      'xcode_settings': {
-        'WARNING_CFLAGS!': [
-          # Don't warn about string->bool used in asserts.
-          '-Wstring-conversion',
-        ],
-      },
-      'cflags!': [
-        '-Wstring-conversion',
-      ],
-    }],
     [ 'OS != "android" and (OS != "linux" or use_cairo==1)', {
       'sources!': [
         'ext/bitmap_platform_device_skia.cc',