Remove the dependency on i18n/icu_string_conversions from base/string_util.h.

      Fix up all files requireing this header to include it directly. Split out the
ICU-dependent string util unit tests into a new file
base/i18n/icu_string_util_unittest.cc

TEST=none
BUG=none
Review URL: http://codereview.chromium.org/269034

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@28674 0039d316-1c4b-4281-b951-d872f2087c98