Don't store ScriptValue in PopStateEvent

Use TraceWrapperV8Reference instead. This CL doesn't remove
custom getter of PopStateEvent.state as more work will be
needed to remove custom bindings.

BUG=501866

Review-Url: https://codereview.chromium.org/2850133002
Cr-Commit-Position: refs/heads/master@{#468565}
4 files changed