bug 1059429 - fix static constructor for FlushEvent r=froydnj

xpcom/base/nsMemoryImpl.h

@@ -41,6 +41,7 @@ public:
 protected:
   struct FlushEvent : public nsIRunnable
   {
+    MOZ_CONSTEXPR FlushEvent() : mReason(nullptr) {}
     NS_DECL_ISUPPORTS_INHERITED
     NS_DECL_NSIRUNNABLE
     const char16_t* mReason;