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Reland "Use the internal copy of gvr_shim in unofficial builds if the internal repository is available."

This change was originally reverted because the included prebuilts
caused crashes in component builds. The prebuilts included with this
change have been updated to fix the component build problem.

Original commit message:
> Use the internal copy of gvr_shim in unofficial builds if the internal repository is available.
>
> Otherwise, link against a new set of gn-built prebuilts.
>
> Bug: 749784
> Change-Id: Id96666125226b3400bb92c31e5c97a7c2a64624d
> Reviewed-on: https://chromium-review.googlesource.com/947577
> Commit-Queue: Peter Collingbourne <pcc@chromium.org>
> Reviewed-by: Brandon Jones <bajones@chromium.org>
> Reviewed-by: agrieve <agrieve@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#546707}

Bug: 749784
Change-Id: I1c7c8c1dc1f96228c81b43d40148be7a54dfcae2
Reviewed-on: https://chromium-review.googlesource.com/1008618Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550757}