Make AW state_serializer handle restoring also legacy format

This ensures that even if an app attempts to restore a state saved
before upgrading WebView (using the new state version) the old version
will still be successfully restored.

BUG=584693

Review URL: https://codereview.chromium.org/1687853002

Cr-Commit-Position: refs/heads/master@{#375418}
3 files changed