Change 4 funcs to JumpList member funcs, retire wstring for jumplist*

This CL seems to have a lot of code changes, but in fact it does
nothing but to change 4 anonymous functions to JumpList class member
functions. Originally these 4 functions were put in an anonymous
namespace, and they are now moved to the end of jumplist.cc. Their
declarations are added in jumplist.h. The body of these 4 functions
have no changes at all. However, one PostTask function call is updated
accordingly due to this change.

Making these 4 functions be member functions of JumpList class allows
them to get access the member variables (which I am going to add) of
the JumpList class. This lays out the foundation for a number of
JumpList improvements I am going to do in parallel. Otherwise, there
will be a lot of duplicated code changes and heavy merge conflicts
ahead. Moreover, this also makes it easier to review the coming CLs on
top of this.

This CL also changes all std::wstring to base::string16 in jumplist.*
and jumplist_updater.* to get rid of presubmit warnings.

BUG=40407, 179576

Review-Url: https://codereview.chromium.org/2870853002
Cr-Commit-Position: refs/heads/master@{#470455}
4 files changed