Add blob serialization support [currently unused] to SerializedScriptValue.
This will be needed by IDB Blob support, in progress.

BUG=108012
R=cmumford,jsbell

Review URL: https://codereview.chromium.org/223123002

git-svn-id: svn://svn.chromium.org/blink/trunk@171190 bbb929c8-8fbe-4397-9dbb-9b2b20218538