Bug 1565515 - Updated comment r=asuth,ttung

The comment referenced the former 3 prefix used for strings, but this is
no longer correct for strings (which now use a 0x30) prefix, and the
function has been generalized to also work for binaries, and got
a parameter aType specifying the prefix. Updated the comment accordingly.

Differential Revision: https://phabricator.services.mozilla.com/D37850

--HG--
extra : moz-landing-system : lando

dom/indexedDB/Key.cpp

@@ -506,7 +506,7 @@ IDBResult<void, IDBSpecialValue::Invalid> Key::EncodeAsString(
     return Exception;
   }
 
-  // The +2 is for initial 3 and trailing 0. We'll compensate for multi-byte
+  // The +2 is for initial aType and trailing 0. We'll compensate for multi-byte
   // chars below.
   uint32_t checkedSize = aEnd - aStart;
   CheckedUint32 size = checkedSize;