Reland "AppCache: Add padding to cross-origin responses."

This is a reland of 85b389caa7d725cdd31f59e9a2b79ff54804b7b7

Initialized CacheRecord::padding_size to 0.

Original change's description:
> AppCache: Add padding to cross-origin responses.
>
> Bug: 918293
> Change-Id: I4f16640f06feac009d6bbbb624951da6d2669f6c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1488059
> Commit-Queue: Staphany Park <staphany@chromium.org>
> Reviewed-by: Victor Costan <pwnall@chromium.org>
> Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#644624}

Bug: 918293
Change-Id: Ie1d3f99c7e8a854d33255a4d66243da2ce16441c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539906
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Staphany Park <staphany@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644719}
 void AppCacheDatabase::ReadEntryRecord(
    const sql::Statement& statement, EntryRecord* record) {
  record->cache_id = statement.ColumnInt64(0);
  record->url = GURL(statement.ColumnString(1));
   record->flags = statement.ColumnInt(2);
   record->response_id = statement.ColumnInt64(3);
   record->response_size = statement.ColumnInt64(4);
  record->padding_size = statement.ColumnInt64(5);
 }

 void AppCacheDatabase::ReadEntryRecord(
    const sql::Statement& statement, EntryRecord* record) {
  record->cache_id = statement.ColumnInt64(0);
  record->url = GURL(statement.ColumnString(1));
   record->flags = statement.ColumnInt(2);
   record->response_id = statement.ColumnInt64(3);
   record->response_size = statement.ColumnInt64(4);
 }
