Hexadecimal escape sequences of the form `\xNN` are more readable
in the context of byte values, and are preferred per Google’s C++
style guide.

https://google.github.io/styleguide/cppguide.html#Non-ASCII_Characters

BUG=v8:7109

Change-Id: I6821ccb804388d99e5601e92fc392afdf496691e
Reviewed-on: https://chromium-review.googlesource.com/803057Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49799}