merge_pr_50260

Validate the same pixel in all 2d.composite meta WPT tests

Having the same assertions in all tests will simplify things when we
migrate these tests to the new test generator, allowing all tests to
share the same common base implementation.

Bug: 40207206
Change-Id: Ifaa73c2b8021ef00290d36729d3f54094951d9d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6192519
Commit-Queue: Jean-Philippe Gravel [email protected]
Reviewed-by: Andres Ricardo Perez [email protected]
Cr-Commit-Position: refs/heads/main@{#1410976}