Remove wchar/wstring version of StringPrintf.
None of actual usage of StringPrintf(wchar_t*) requires wchar support.
All of them can be replaced by char version of StringPrintf, with
conversion to wstring by ASCIIToWide().
The original intent of this CL is because Android doesn't support wide
characters. But Android suppplies a header wchar.h without a real
implementation, which makes code compile on Android. 4 base_unittests
fail on Android.