Fix bug 164363 "nsDiskCacheEntry::Size() is incorrect" r=sdagley, sr=darin.

netwerk/cache/src/nsDiskCacheEntry.h

@@ -53,10 +53,8 @@ struct nsDiskCacheEntry {
     char            mKeyStart[1];   // start of key data
     //              mMetaDataStart = mKeyStart[mKeySize];
 
-    PRUint32        Size()    { return sizeof(nsDiskCacheEntry)
-                                     - sizeof(char) // subtract default key size
-                                     + mKeySize     // plus actual key size
-                                     + mMetaDataSize;
+    PRUint32        Size()    { return offsetof(nsDiskCacheEntry,mKeyStart) + 
+                                    mKeySize + mMetaDataSize;
                               }
 
     nsCacheEntry *  CreateCacheEntry(nsCacheDevice *  device);