Rename GooString::getCString GooString::c_str

This is the name used by std::string.
5 jobs for replace-getcstring-by-c-str in 15 minutes and 31 seconds (queued for 3 minutes and 32 seconds)
Status Job ID Name Coverage
  Build
passed #42651
build

00:09:20

passed #42652
build_clang_libcpp

00:08:34

passed #42655
build_clazy

00:05:24

passed #42653
build_gcc_49

00:09:40

passed #42654
build_mingw64_fedora

00:04:50