Fix a misuse of pragma disable/default in a header file.

This may override the enable/disable of this warning in an other file which include base_helpers.h.

R=ajwong@chromium.org, erikwright@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/20353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@214573 0039d316-1c4b-4281-b951-d872f2087c98

base/bind_helpers.h

@@ -237,12 +237,13 @@ class SupportsAddRefAndRelease {
 // common pattern for refcounted types. It does this even though no attempt to
 // instantiate Base is made.  We disable the warning for this definition.
 #if defined(OS_WIN)
+#pragma warning(push)
 #pragma warning(disable:4624)
 #endif
   struct Base : public T, public BaseMixin {
   };
 #if defined(OS_WIN)
-#pragma warning(default:4624)
+#pragma warning(pop)
 #endif
 
   template <void(BaseMixin::*)(void)> struct Helper {};