Bug 1881863 - Part 3: Use the XPIDL-generated PRTime typedef, r=xpcom-reviewers,emilio

The typedef for PRTime generated from XPIDL is being `#if 0`-ed out in
`nsrootidl.idl`, however was also declared earlier within a `%{C++` block.
Remove the `#if 0` to allow the XPIDL-generated typedef to be generated and
used instead, for better consistency between target languages.

Differential Revision: https://phabricator.services.mozilla.com/D206384

xpcom/base/nsrootidl.idl

@@ -11,7 +11,6 @@
 
 #include "nscore.h"
 #include "nsID.h"
-typedef int64_t PRTime;
 
 /*
  * Forward declarations for new string types
@@ -29,10 +28,6 @@ class Promise;
 } // namespace dom
 } // namespace mozilla
 
-/*
- * Start commenting out the C++ versions of the below in the output header
- */
-#if 0
 %}
 
 typedef long long PRTime;
@@ -85,10 +80,3 @@ typedef long long PRTime;
               native jsid(jsid);
 
 [ptr, promise] native Promise(ignored);
-
-%{C++
-/*
- * End commenting out the C++ versions of the above in the output header
- */
-#endif
-%}