Add Precache.Fetch.ResponseBytes.Daily.

components/precache/core/proto/quota.proto

 // Copyright 2013 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.
 
 syntax = "proto2";
 
 package precache;
 
 // Chrome requires this.
 option optimize_for = LITE_RUNTIME;
 
 // Quota limit and expiry time. This is stored in a database, and not
 // transferred via network.
 message PrecacheQuota {
-  // Represents the start time of this quota.
+  // Represents the start time of this quota. After enough time has elapsed
+  // since the start time (as defined in PrecacheFetcher::IsQuotaTimeExpired), a
+  // new quota is created.
   optional int64 start_time = 1;
 
   // Maximum number of bytes that can be fetched until this quota expires.
+  // Initialized to PrecacheConfigurationSettings.daily_quota_total and
+  // decremented for every byte downloaded. After this reaches zero, the
+  // PrecacheFetcher will not download any more until a new quota window starts.
   optional uint64 remaining = 2;
-};
+};