We should only ever call value() on a variable while we are inside a block.
This CL adds a DEBUG check to this effect.

Bug: 
Change-Id: Ic85fae70e2c3543ff79e3234ba26e1daa234f7e3
Reviewed-on: https://chromium-review.googlesource.com/772233Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49492}