Add Precache.Fetch.ResponseBytes.Daily.

components/precache/core/proto/quota.proto

Index: components/precache/core/proto/quota.proto
diff --git a/components/precache/core/proto/quota.proto b/components/precache/core/proto/quota.proto
index effb895e75cc5a42865913ccac3d90dec379655f..6a60e7c54c6ec9947b2acd03ad531a6e1fecb2fd 100644
--- a/components/precache/core/proto/quota.proto
+++ b/components/precache/core/proto/quota.proto
@@ -12,9 +12,14 @@ 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;
-};
+};