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}
bool AppCacheDatabase::CreateSchema() {
  sql::Transaction transaction(db_.get());
  if (!transaction.Begin())
    return false;

  if (!meta_table_->Init(db_.get(), kCurrentVersion, kCompatibleVersion))
    return false;

  if (!meta_table_->SetValue(kExperimentFlagsKey,
                             GetActiveExperimentFlags())) {
    return false;
  }

  for (int i = 0; i < kTableCount; ++i) {
    if (!CreateTable(db_.get(), kTables[i]))
      return false;
  }

  for (int i = 0; i < kIndexCount; ++i) {
    if (!CreateIndex(db_.get(), kIndexes[i]))
      return false;
  }

  return transaction.Commit();
 }

bool AppCacheDatabase::CreateSchema() {
  sql::Transaction transaction(db_.get());
  if (!transaction.Begin())
    return false;

  if (!meta_table_->Init(db_.get(), kCurrentVersion, kCompatibleVersion))
    return false;

  if (!meta_table_->SetValue(kExperimentFlagsKey,
                             GetActiveExperimentFlags())) {
    return false;
  }

  for (int i = 0; i < kTableCount; ++i) {
    if (!CreateTable(db_.get(), kTables[i]))
      return false;
  }

  for (int i = 0; i < kIndexCount; ++i) {
    if (!CreateIndex(db_.get(), kIndexes[i]))
      return false;
  }

  return transaction.Commit();
 }
