GitLab

The decoder blithely passed any old 16-bit code unit through, in
violation of the Encoding standard. Surrogate pairs should go through
unscathed:

  [ ... 0xD800 0xDC00 ... ] => [ ... U+D800 U+DC00 ... ]

But cases like these should result in replacement characters:

  [ ... 0xD800 ... ] => [ ... U+FFFD ... ]
  [ ... 0xDC00 ... ] => [ ... U+FFFD ... ]
  [ ... 0xDC00 0xD800 ... ] => [ ... U+FFFD U+FFFD ... ]

This aligns Chrome's behavior with Firefox and Edge.

Note that flushing at the end of a stream remains a special case.
Streams terminating in the above sequences will not get replacements
emitted (current behavior). In addition, a lead surrogate appearing at
the end of a stream will now not be emitted, matching other browsers.

BUG=368904
R=jshin@chromium.org,foolip@chromium.org

Review-Url: https://codereview.chromium.org/2379333003
Cr-Commit-Position: refs/heads/master@{#422929}