Use std::unique_ptr for m_formData

To make it clear that it is an owning pointer.
6 jobs for use-unique-ptr-for-formdata in 3 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1226751
build

00:02:29

passed #1226756
build_android

00:00:46

passed #1226752
build_clang_libcpp

00:03:28

passed #1226755
build_clazy

00:02:11

passed #1226754
build_mingw64_fedora

00:02:02

passed #1226753
build_ubuntu_16_04

00:01:45