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}
AppCacheUpdateJob::AppCacheUpdateJob(AppCacheServiceImpl* service,
                                     AppCacheGroup* group)
    : service_(service),
      manifest_url_(group->manifest_url()),
      group_(group),
      update_type_(UNKNOWN_TYPE),
      internal_state_(FETCH_MANIFEST),
      doing_full_update_check_(false),
      master_entries_completed_(0),
      url_fetches_completed_(0),
      manifest_fetcher_(nullptr),
      manifest_has_valid_mime_type_(false),
      stored_state_(UNSTORED),
      storage_(service->storage()),
      weak_factory_(this) {
  service_->AddObserver(this);
}

AppCacheUpdateJob::AppCacheUpdateJob(AppCacheServiceImpl* service,
                                     AppCacheGroup* group)
    : service_(service),
      manifest_url_(group->manifest_url()),
      group_(group),
      update_type_(UNKNOWN_TYPE),
      internal_state_(FETCH_MANIFEST),
      doing_full_update_check_(false),
      master_entries_completed_(0),
      url_fetches_completed_(0),
      manifest_fetcher_(nullptr),
      manifest_has_valid_mime_type_(false),
      stored_state_(UNSTORED),
      storage_(service->storage()),
      weak_factory_(this) {
  service_->AddObserver(this);
}
