Bug 1379564 - Make BaseTimeDuration::Forever() constexpr. r=birtles

MozReview-Commit-ID: DFse9SD1taP

--HG--
extra : rebase_source : a23d92c8b959c7f680b06c223e266ff1646813b0

mozglue/misc/TimeStamp.h

@@ -138,7 +138,7 @@ public:
     return FromMilliseconds(aMicroseconds / 1000.0);
   }
 
-  static BaseTimeDuration Forever()
+  static constexpr BaseTimeDuration Forever()
   {
     return FromTicks(INT64_MAX);
   }