Because we want to move all validation to the API layer, we need to
move the index validation to the API layer. This means exposing the
index cache to the validation layer.

In the future we will probably want a way to skip index validation
when it's not necessary, or the chosen back-end handles index
validation on its own.

BUG=angle:571

Change-Id: Iee1618e669bc28abf7e0709ca4a03b4727b1383e
Reviewed-on: https://chromium-review.googlesource.com/210646Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>