If we're hashing a single sequential string we don't need the state that the
string hasher itself tracks. This also drops first_char since we can simply
check that array_index is still 0.

Change-Id: Icb69709267426358f7c301eeb45936843ba261b0
Reviewed-on: https://chromium-review.googlesource.com/c/1340258
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57635}