Fix two byte string-search on big endian platforms
Fix two byte string-search on big endian platforms

Use AlignDown instead of IsAligned to avoid false negatives
on big endian platforms
Use byte with highest value to speedup search

BUG=

Review URL: https://codereview.chromium.org/1324803003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30615}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh