Check that the PNaCl cache hash is truly derived from the bitcode content.

ppapi/api/private/ppb_nacl_hash_private.idl

+/* Copyright (c) 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.
+ */
+
+/* This file contains NaCl private interfaces for hashing. This interface
+ * is not versioned and is for internal Chrome use. It may change without
+ * notice. If we convert the NaCl plugin from PPAPI to be renderer code
+ * then we can depend on the chrome hash functions directly.
+ */
+
+/* PPB_NaCl_Hash_Private */
+interface PPB_NaCl_Hash_Private {
+  // Create a hasher object for incremental hashing.
+  mem_t CreateSHA256Hasher();
+
+  // Update the hash with len more bytes of data.
+  void Update([inout] mem_t hasher, [in] mem_t data, uint32_t len);
+
+  // Finish hashing and get a digest up to len bytes.
+  void Finish([inout] mem_t hasher, [out] mem_t output, [in] uint32_t len);
+
+  // Delete the hasher object.
+  void Delete([in] mem_t hasher);
+
+  // Compare two hashes and return PP_TRUE if they are equal.
+  PP_Bool SecureMemEqual([in] mem_t hash1, [in] mem_t hash2,
+                         [in] uint32_t len);
+};