Update and re-enable set-breakpoint-on-blocking-main-thread web test

This web test was outdated (using SharedArrays without setting
the required CORS headers, and old breakpoint sidebar).

This updates the test to correctly set the headers and use the
new breakpoint sidebar pane, and re-enables it.

Fixed: 40175672
Change-Id: I4e902baf009ce8562e31b94b4b890357393b8150
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6620926
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Auto-Submit: Kim-Anh Tran <kimanh@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1469860}