Bug 1411027 - avoid using memset on a not-trivial type like nsTabSizes. r=froydnj

MozReview-Commit-ID: 7bFwF4fs4gj

--HG--
extra : rebase_source : 23ec6f4a3fcfef987338d192577bd8cf3c0dbcba

dom/base/nsWindowSizes.h

@@ -19,7 +19,12 @@ public:
       Other       // Everything else.
   };
 
-  nsTabSizes() { mozilla::PodZero(this); }
+  nsTabSizes()
+    : mDom(0)
+    , mStyle(0)
+    , mOther(0)
+  {
+  }
 
   void add(Kind kind, size_t n)
   {