consume() (the no argument version) does a bunch of work handling out of bounds
access, NUL replacement characters, and returning the char. Many of the callers
just want to increment the offset. We could do that by replacing the call sites
with consume(1) which is the "fast" version that doesn't have a return value,
but it'd make the code more clear to just inline advance() in the right spots.

BUG=605792

Review-Url: https://codereview.chromium.org/2099413002
Cr-Commit-Position: refs/heads/master@{#402328}