Bug 1409382 - EnumSet.h - Initialize mVersion to silence a warning with gcc 8 r=froydnj

MozReview-Commit-ID: CfTs6IAP1z5

--HG--
extra : rebase_source : 80051fe498d20e08060afca577a6abc4ee3c35cd

mfbt/EnumSet.h

@@ -335,7 +335,7 @@ private:
   uint32_t mBitField;
 
 #ifdef DEBUG
-  uint64_t mVersion;
+  uint64_t mVersion = 0;
 #endif
 };