MSVC noted the unsigned |n| = size_t |blocks| could be a possible
loss in precision. No loss in precision occurs since (n > blocks)
at this point: |blocks| fits in an unsigned type.

To silence compiler warnings, first update BUILD.gn for the adler
SIMD code to use chromium compiler:chromium_code rule (more error
checking), rather than the permissive "compiler:no_chromium_code"
rule. Then cast |blocks| to unsigned on assigment to |n| (this is
safe to do as mentioned above).

No change in behavior, no new tests.

Tbr: cblume@chromium.org
Bug: 762564
Change-Id: Ia97120bcca206287fd42b97674f8a6215283e4a5
Reviewed-on: https://chromium-review.googlesource.com/835927


Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#525285}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0cb9a22e2fb55e092342192d66f7e33c14432d27