[Sync] Add size and crc32c to AttachmentId.

sync/api/attachments/attachment_metadata_unittest.cc

 // Copyright 2014 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.
 
 #include "sync/api/attachments/attachment_metadata.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace syncer {
 
 class AttachmentMetadataTest : public testing::Test {};
 
 TEST_F(AttachmentMetadataTest, Create) {
-  AttachmentId id = AttachmentId::Create();
   size_t size = 42;
+  uint32_t crc32c = 2349829;
+  AttachmentId id = AttachmentId::Create(size, crc32c);
   AttachmentMetadata metadata(id, size);
   EXPECT_EQ(metadata.GetId(), id);
   EXPECT_EQ(metadata.GetSize(), size);
 }
 
 }  // namespace syncer