Remove some uninitialized reads from simple cache metadata serializer.

net/disk_cache/simple/simple_disk_format.h

Index: net/disk_cache/simple/simple_disk_format.h
diff --git a/net/disk_cache/simple/simple_disk_format.h b/net/disk_cache/simple/simple_disk_format.h
index 18dfe9661ad00402c6407254dc58e831d4e48e33..062ff13bbd64237f4ea0d998d676a52a469d2b5b 100644
--- a/net/disk_cache/simple/simple_disk_format.h
+++ b/net/disk_cache/simple/simple_disk_format.h
@@ -53,6 +53,7 @@ std::string GetEntryHashForKey(const std::string& key);
 namespace SimpleIndexFile {
   // Simple Index File metadata is defined here.
   struct Header {
+    Header();
     uint64 initial_magic_number;
     uint32 version;
     uint64 number_of_entries;
@@ -89,6 +90,7 @@ namespace SimpleIndexFile {
   const size_t kEntryMetadataSize = sizeof(EntryMetadata);
 
   struct Footer {
+    Footer();
     uint32 crc;
   };