Fix "unknown argument" message from clang plugin on ChromeOS build.

BUG=123295
TEST=see the build log.

Review URL: https://chromiumcodereview.appspot.com/10392194

git-svn-id: http://src.chromium.org/svn/trunk/src/build@138229 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -1099,7 +1099,7 @@
               '-Xclang',
               '-plugin-arg-find-bad-constructs',
               '-Xclang',
-              '-skip-refcounted-dtors'
+              'skip-refcounted-dtors'
             ],
           }, {
             'clang_chrome_plugins_flags': [