Update HpackHeaderTable class member comments.

This CL lands the third_party/spdy portion of server change 211551817 by
bnc.

BUG=488484

Change-Id: I7090d3fcc9ec885114ff5674fd0bba531258130c
Reviewed-on: https://chromium-review.googlesource.com/c/1252086
Commit-Queue: Yang Song <yasong@chromium.org>
Reviewed-by: Yang Song <yasong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596195}

net/third_party/spdy/core/hpack/hpack_header_table.h

@@ -142,8 +142,10 @@ class SPDY_EXPORT_PRIVATE HpackHeaderTable {
   // Evicts |count| oldest entries from the table.
   void Evict(size_t count);
 
-  // |static_entries_| and |static_index_| are owned by HpackStaticTable
-  // singleton.
+  // |static_entries_|, |static_index_|, and |static_name_index_| are owned by
+  // HpackStaticTable singleton.
+
+  // Tracks HpackEntries by index.
   const EntryTable& static_entries_;
   EntryTable dynamic_entries_;