It's not obvious for Utf16CharacterStream::ReadBlock that the block it
has to read is implicitly the one at the current pos(), and it was
slightly odd how ReadBlockAt had to fiddle with buffer_* values to make
pos() return the desired value before ReadBlock is called -- especially
since ReadBlock would usually overwrite those changes.

Instead, we can just make ReadBlock take an explicit position, and get
rid of ReadBlockAt entirely.

As a drive-by, I was always confused by what the various buffer_*
actually mean (especially the difference between buffer_cursor_ and
buffer_pos_) so document them with some ASCII art.

Change-Id: I610019089920692f54e01ae979c0ba827779e414
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3217194
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77315}