This CL adds a DCHECK to make the uses() function safer: we store what
the next pointer should be to comparing with the next use (from the 
current element in the linked list). This helps detect code which 
invalidates the use-list iterator, which would otherwise cause 
hard-to-debug errors.

Bug: 
Change-Id: I3875361809ffd55fb8be2cbb15af3250e3fd4c12
Reviewed-on: https://chromium-review.googlesource.com/596030Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Alexandre Talon <alexandret@google.com>
Cr-Commit-Position: refs/heads/master@{#47074}