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Add a common ScriptContext getter to retrieve a ScriptContext from a
given v8::Context that works across threads. This previously existed,
but only in native_extension_bindings_system.cc. Pull it out to a common
file to a) make it reusable from other consumers (custom hooks will need
to use this) and b) remove (direct) threading knowledge from the bindings
system.

Bug: 653596

Change-Id: I09a0e4ea76bb891ab1871ca1deb0d315e6e9789d
Reviewed-on: https://chromium-review.googlesource.com/812173Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522243}