Fix performance issue in RAW mode

While investigating the use of RAW mode, I noticed that it was *slower*
for compression than GZIP or ZLIB wrapper formats, which didn't make
sense (i.e. we don't calculate a data integrity check using RAW mode).

It turns out that the code was falling back to the portable implementation
of insert_string(), instead of using the optimized version that rely on 'crc32w'
as a hash function.

The reason is that CPU features detection is not triggered while using
RAW mode (i.e. we never call crc32() with a NULL buffer).

This patch fixes this issue ensuring that RAW mode is going to be faster
for both compression/decompression than either ZLIB or GZIP formats.

Bug: 833361
Change-Id: I285297f67ffc0114700ed03c2186ad21aab8b40e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929634


Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#717877}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 171a0a69eb5d70f8a9f44000e26bc7dc65f1fd97