Use a direct include of strings headers in testing/, third_party/, tools/.

BUG=247723
TEST=none
TBR=ben@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15851009

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@205470 4ff67af0-8c30-449e-8e8b-ad334ec8d88c