[Extensions Bindings] Add a ScriptContext::context_id

Add a context_id (and accessor) to ScriptContext that uniquely
identifies that script context (globally, across all renderer
processes) using UnguessableToken. This is already needed in messaging
bindings, but could be useful other places as well (event bookkeeping).
This also avoids the need to redefine logic to uniquely identify a
context for native messaging bindings.

Bug: 653596
Change-Id: Iffd11f97a6e645a6f31cddead4e06eeb964bb258
Reviewed-on: https://chromium-review.googlesource.com/668678
Reviewed-by: Istiaque Ahmed <[email protected]>
Commit-Queue: Devlin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#502361}
8 files changed