Update stream web_tests README

The README file for chromium-specific stream web tests made reference to
the SimpleQueue class, which no longer exists. Update it to reflect the
current situation.

Change-Id: Iea48716869617d2963dc2e7a13c465a83d44da44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644417
Commit-Queue: Adam Rice <ricea@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Auto-Submit: Adam Rice <ricea@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845954}

third_party/blink/web_tests/http/tests/streams/chromium/README.txt

-These tests exercise Chromium/Blink specific behavior (like garbage
-collection, not crashing, etc) and will not be upstreamed to the
-W3C web-platform-tests.
+These tests exercise Chromium/Blink specific behavior (like garbage collection,
+not crashing, etc) and will not be upstreamed to the W3C web-platform-tests.
 
-The simple-queue-*.html tests test the SimpleQueue class which is used
-internally to Chrome's Stream API implementation. It uses nodes of 16K
-elements. These tests exercise the code paths which create and switch
-nodes. They are split into separate files because the large number of
-iterations cause timeouts on MSAN bots.
+The simple-queue-*.html tests originally tested the SimpleQueue class which was
+used internally to Blink's Stream API JavaScript implementation. The
+implementation has since been replaced, but the tests are retained to prevent
+regressions.