Port experiment_labels.h to be cross-platform.

The experiment label utility functions are used on iOS as well as windows.  This
CL removes uses of wchar_t (which is not portable).

BUG=316584
TEST=No visible impact.

Review URL: https://codereview.chromium.org/106093003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239223 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239223