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

chrome/browser/extensions/settings/syncable_settings_storage.cc

Index: chrome/browser/extensions/settings/syncable_settings_storage.cc
diff --git a/chrome/browser/extensions/settings/syncable_settings_storage.cc b/chrome/browser/extensions/settings/syncable_settings_storage.cc
index f65d7e3ed94946a7e9550d439f414bf84e7a57e6..b7ca63a6810a4b643d240b51b24d25fa2364463e 100644
--- a/chrome/browser/extensions/settings/syncable_settings_storage.cc
+++ b/chrome/browser/extensions/settings/syncable_settings_storage.cc
@@ -32,6 +32,11 @@ 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));