Review Board 3.0 introduced a more efficient way of transferring diff

fragments for comments, and a mechanism for providing updates to entries

on the review request page. These used a streamable format that encoded

byte lengths followed by content.
The problem is, JavaScript doesn't deal with strings as binary data,

they deal with them as Unicode data. This meant that our byte counts

would be wildly off whenever any multi-byte characters were introduced

in any of the content sections, causing various breakages.
A long-term (and in-progress) solution is to switch to interpreting

these payloads as binary content by using JavaScript's ArrayBuffer and

DataView objects, which would allow any form of content to be made

available in the payload. For now, though, since these are all UTF-8

string content, we can resolve this issue by storing character counts

and not byte counts.

Python and JavaScript unit tests pass on Chrome, Firefox, and IE.
Manually tested Unicode content in updates and diff fragments. They

loaded correctly.