This is in preparation for upcmoming scanner + bookmarking cleanups.

Also, drive-by fix for setting a bookmark close to the end of the stream,
when the look-ahead character (c0_) is kEndOfInput, which the bookmarking
logic also used as kNoBookmark.

R=marja@chomium.org
BUG=v8:4947

Review-Url: https://codereview.chromium.org/2345053003
Cr-Commit-Position: refs/heads/master@{#39507}