Add UsageDetails dictionary.

storage.bs

@@ -178,6 +178,7 @@ larger <a>site storage quota</a>. Factors such as navigation frequency, recency
 bookmarking, and <a href="#persistence">permission</a> for {{"persistent-storage"}} can be used as
 indications of "popularity".
 
+A <dfn export>site storage backend</dfn> is a specific site storage implementation.
 
 
 <h2 id=ui-guidelines>User Interface Guidelines</h2>
@@ -234,7 +235,16 @@ interface StorageManager {
 dictionary StorageEstimate {
   unsigned long long usage;
   unsigned long long quota;
+  StorageUsageDetails usageDetails;
 };
+
+dictionary StorageUsageDetails {
+    unsigned long long applicationCache;
+    unsigned long long caches;
+    unsigned long long indexedDB;
+    unsigned long long serviceWorkerRegistrations;
+};
+
 </pre>
 
 The <dfn method for=StorageManager><code>persisted()</code></dfn> method, when invoked, must run
@@ -331,8 +341,13 @@ must run these steps:
 
    <li><p>Let <var>quota</var> be <a>site storage quota</a> for <var>origin</var>.
 
+   <li><p>Let <var>usageDetails</var> be a new {{StorageUsageDetails}} dictionary
+  whose members are the <a>site storage usage</a> for each <a>site storage backend</a>
+  for which the <a>site storage usage</a> is greater than 0.
+
    <li><p>Let <var>dictionary</var> be a new {{StorageEstimate}} dictionary whose {{usage}} member
-   is <var>usage</var> and {{quota}} member is <var>quota</var>.
+   is <var>usage</var>, {{quota}} member is <var>quota</var> and {{usageDetails}}
+   member is <var>usageDetails</var>.
 
    <li>
     <p>If there was an internal error while obtaining <var>usage</var> and <var>quota</var>, then