Extension Storage API: expose storage quota information to extensions, via:

chrome/browser/extensions/settings/weak_unlimited_settings_storage.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/extensions/settings/weak_unlimited_settings_storage.h"
 
 namespace extensions {
 
 WeakUnlimitedSettingsStorage::WeakUnlimitedSettingsStorage(
     SettingsStorage* delegate)
     : delegate_(delegate) {}
 
 WeakUnlimitedSettingsStorage::~WeakUnlimitedSettingsStorage() {}
 
+size_t WeakUnlimitedSettingsStorage::GetBytesInUse() {
+  return delegate_->GetBytesInUse();
+}
+
 SettingsStorage::ReadResult WeakUnlimitedSettingsStorage::Get(
     const std::string& key) {
   return delegate_->Get(key);
 }
 
 SettingsStorage::ReadResult WeakUnlimitedSettingsStorage::Get(
     const std::vector<std::string>& keys) {
   return delegate_->Get(keys);
 }
 
@@ skipping 19 matching lines @@
 SettingsStorage::WriteResult WeakUnlimitedSettingsStorage::Remove(
     const std::vector<std::string>& keys) {
   return delegate_->Remove(keys);
 }
 
 SettingsStorage::WriteResult WeakUnlimitedSettingsStorage::Clear() {
   return delegate_->Clear();
 }
 
 }  // namespace extensions