Bug 1719396: Don't hash sizeof(size_t) in HashBytes r=glandium

The loop in HashBytes currently hashes both a word of data (as intended) and the length of that word of data (a constant).

This was [acknowledged as a bug](https://bugzilla.mozilla.org/show_bug.cgi?id=729940#c49) when first written, but slipped through the cracks and was never fixed.

Differential Revision: https://phabricator.services.mozilla.com/D119466

mfbt/HashFunctions.cpp

@@ -24,7 +24,7 @@ uint32_t HashBytes(const void* aBytes, size_t aLength) {
     size_t data;
     memcpy(&data, b + i, sizeof(size_t));
 
-    hash = AddToHash(hash, data, sizeof(data));
+    hash = AddToHash(hash, data);
   }
 
   /* Get the remaining bytes. */