I loosened the DCHECKs here but I think they are still fundamentally
safe: `length` must be <= the actual length of the source (so that
there are actually enough elements to copy), and `length` must also be
<= the destination length, minus the offset (so there is enough space
to copy the elements into).

Bug: chromium:816317
Change-Id: Ice00ac60f4884363f6065ffee71f6ab1d1b32dbc
Reviewed-on: https://chromium-review.googlesource.com/937209Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51566}