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

chrome/browser/extensions/settings/syncable_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/syncable_settings_storage.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/extensions/settings/settings_namespace.h"
 #include "chrome/browser/extensions/settings/settings_sync_util.h"
 #include "chrome/browser/sync/api/sync_data.h"
@@ skipping 14 matching lines @@
       delegate_(delegate),
       sync_type_(syncable::UNSPECIFIED),
       sync_processor_(NULL) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
 }
 
 SyncableSettingsStorage::~SyncableSettingsStorage() {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
 }
 
+size_t SyncableSettingsStorage::GetBytesInUse() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
+  return delegate_->GetBytesInUse();
+}
+
 SettingsStorage::ReadResult SyncableSettingsStorage::Get(
     const std::string& key) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
   return delegate_->Get(key);
 }
 
 SettingsStorage::ReadResult SyncableSettingsStorage::Get(
     const std::vector<std::string>& keys) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
   return delegate_->Get(keys);
@@ skipping 390 matching lines @@
         FROM_HERE,
         std::string("Error pushing sync remove to local settings: ") +
             result.error(),
         sync_type_);
   }
   changes->push_back(SettingChange(key, old_value, NULL));
   return SyncError();
 }
 
 }  // namespace extensions