release 2019-04-01
Get rid of `using std::string;`. (part 7 of 7) Change-Id: I2df2ec398bb0a5d9ac9019f575c494107a82fbe3 Reviewed-on: https://code-review.googlesource.com/c/re2/+/39136 Reviewed-by: Paul Wankadia <junyer@google.com>