This is straightforward, with only two notable things.

- `#include "nsXPIDLString.h" is replaced with `#include "nsString.h"`
  throughout, because all nsXPIDLString.h did was include nsString.h. The
  exception is for files which already include nsString.h, in which case the
  patch just removes the nsXPIDLString.h inclusion.

- The patch removes the |xpidl_string| gtest, but improves the |voided| test to
  cover some of its ground, e.g. testing Adopt(nullptr).

--HG--
extra : rebase_source : 452cc4a08046a1adb1a8099a7e85a1917de5add8