Fix DCHECK on SetBookmark.

Fix DCHECK on SetBookmark.

The DCHECK was a lie. The idea was that - when a bookmark is set -
the scanner must clearly be at a character boundary and hence the
bookmark does not need to save a 'partial' UTF-8 code point. The
first part is true - the Scanner is always at a character boundary -
but the 'partial' UTF-8 code point is at the end of a block, not at
the current character position of the Scanner.
Hence, the 'partial' character needs to be saved as well.

jkummerow: Thanks for noticing.

BUG=chromium:470930
R=jochen@chromium.org, jkummerow@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1154773004

Cr-Commit-Position: refs/heads/master@{#28661}