Enable zlib's assertions in dcheck_always_on

Those assertions fill the same role as our DCHECKs. Having them enabled
more would surface more issues (such as crbug.com/1511987) quicker.

Bug: 1511987
Change-Id: Ic0e277c82c93379103f32900f0f4feec6d09655e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5125959


Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1238258}
NOKEYCHECK=True
GitOrigin-RevId: 80fab0f77a40cfcaf44736b307369123df4004f0