[Extensions] Check is_valid() in app bindings
[Extensions] Check is_valid() in app bindings

The AppBindings object can potentially receive an IPC message after it's
been invalided. Check before entering the v8 context.

BUG=610224

Review-Url: https://codereview.chromium.org/1960173002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#392367}

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