Bug 1353593 - Part 1: Allow implicit conversion of non-const char16ptr_t to wchar*. r=froydnj

mfbt/Char16.h

@@ -56,6 +56,12 @@ public:
   {
     return reinterpret_cast<const wchar_t*>(mPtr);
   }
+
+  operator wchar_t*()
+  {
+    return const_cast<wchar_t*>(reinterpret_cast<const wchar_t*>(mPtr));
+  }
+
   operator const void*() const
   {
     return mPtr;