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

third_party/WebKit/public/web/WebSerializedScriptValueVersion.h

Index: third_party/WebKit/public/web/WebSerializedScriptValueVersion.h
diff --git a/third_party/WebKit/public/web/WebSerializedScriptValueVersion.h b/third_party/WebKit/public/web/WebSerializedScriptValueVersion.h
index a808ae23fedda2fd79b7392ff8e258f50049c2c9..9a4b3d26c06c4068b1dfb2966dff9eb2d08f9293 100644
--- a/third_party/WebKit/public/web/WebSerializedScriptValueVersion.h
+++ b/third_party/WebKit/public/web/WebSerializedScriptValueVersion.h
@@ -31,14 +31,29 @@
 #ifndef WebSerializedScriptValueVersion_h
 #define WebSerializedScriptValueVersion_h
 
+#include <cstdint>
+
+#include "third_party/WebKit/public/platform/WebCommon.h"
+
 namespace blink {
 
-// SerializedScriptValue wire format version. Versions higher than
-// this value will not be deserializable by WebSerializedScriptValue.
-// Embedders may serialize this as out-of-band metadata along with
-// collections of serialized data so that version skew can be detected
-// before deserializing individual values.
-const unsigned kSerializedScriptValueVersion = 16;
+// The currently supported version data for serialized script values.
+// This is the version that will be written. Values can be read only if they
+// were written by a serializer for which both |v8_version| and |blink_version|
+// were less than or equal to the current version.
+//
+// Versions higher than |current()| will not be deserializable by
+// WebSerializedScriptValue.  Embedders may serialize this as out-of-band
+// metadata along with collections of serialized data so that version skew can
+// be detected before deserializing individual values.
+//
+// In general, both versions should monotonically increase.
+struct WebSerializedScriptValueVersion {
+  uint32_t v8Version = 0;

[cmumford, 2017/04/05 21:23:18]

v8 has a major and minor version (V8_MAJOR_VERSION & V8_MINOR_VERSION). How will
they map to v8Version?

[jbroman, 2017/04/05 21:27:10]

V8 has a wire format version, which is analogous to Blink's. I recently exposed
it as v8::ValueSerializer::GetCurrentDataFormatVersion(), though that's only
available at runtime, so it'd have to move to be a header constant to satisfy
the concerns you and dcheng have.

The V8 published version number has to do with the V8 release process, and isn't
great for this purpose. (Similarly, we don't use the Chromium
milestone.minor.branch.patch version for the Blink-side versioning.)

+  uint32_t blinkVersion = 0;
+
+  BLINK_EXPORT static WebSerializedScriptValueVersion current();
+};
 
 }  // namespace blink