Switch text_codec_utf8.* to "blink" namespace

* Adds some temporary `using WTF::*` to avoid unnecessary code changes.
* Renames TextCodecUTF8 to TextCodecUtf8 to comply with Google C++
  coding style.

This CL has no behavior changes.

Bug: 422768753
Change-Id: I27dd60160d3c9d803f0d699a4b313a08853cdad9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6637579
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1473463}