Implement fillGooString in terms of fillString

This requires to add a method to GooString that allows access
as a mutable std::string&.
6 jobs for use-string-values-in-linkactions in 3 minutes and 47 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1491923
build

00:03:44

passed #1491928
build_android

00:01:03

passed #1491924
build_clang_libcpp

00:03:23

passed #1491927
build_clazy

00:02:18

passed #1491926
build_mingw64_fedora

00:03:47

passed #1491925
build_ubuntu_16_04

00:01:10