Use a two-part data format version in IndexedDB metadata.

content/browser/indexed_db/indexed_db_data_format_version.cc

Index: content/browser/indexed_db/indexed_db_data_format_version.cc
diff --git a/content/browser/indexed_db/indexed_db_data_format_version.cc b/content/browser/indexed_db/indexed_db_data_format_version.cc
new file mode 100644
index 0000000000000000000000000000000000000000..b8a363cea5a842e5fef824103704e14158845297
--- /dev/null
+++ b/content/browser/indexed_db/indexed_db_data_format_version.cc
@@ -0,0 +1,16 @@
+// Copyright 2017 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 "content/browser/indexed_db/indexed_db_data_format_version.h"
+
+#include "third_party/WebKit/public/web/WebSerializedScriptValueVersion.h"
+
+namespace content {
+
+// static
+IndexedDBDataFormatVersion IndexedDBDataFormatVersion::current_(
+    0,
+    blink::kSerializedScriptValueVersion);

[dcheng, 2017/04/06 21:14:19]

Is there a guarantee that this will avoid runtime initialization?

[jbroman, 2017/04/06 21:26:25]

The type has a constexpr constructor, so it's certainly permitted to avoid it.
I've checked that at least gcc and clang do (indeed it's _really_ easy for them
to). It's a little more work for me to check MSVC, but I'd be surprised if it
didn't do the right thing here.

Technically I'm not sure the language forbids compilers from putting runtime
initialization for any variable with static initialization if it wants, so I
don't know how I can guarantee this other than saying that I've checked two
compilers, and I'd expect the sizes alarm to go off if I'm wrong.

I can't make the whole thing constexpr because I actually want this to be a
mutable static field, for tests. But this should compile down to the obvious
thing (8 bytes in the .data section or equivalent).

+
+}  // namespace content