build: run browsers tests on chromium locally (#38435)

Previously we added a browser target for `firefox` into the
dev-infra package. It looks like as part of this change, we
accidentally switched the local web testing target to `firefox`.

Web tests are not commonly run locally as we use Domino and
NodeJS tests for primary development. Sometimes though we intend
to run tests in a browser. This would currently work with Firefox
but not on Windows (as Firefox is a noop there in Bazel).

This commit switches the primary browser back to `chromium`. Also
Firefox has been added as a second browser to web testing targets.

This allows us to reduce browsers in the legacy Saucelabs job. i.e.
not running Chrome and Firefox there. This should increase stability
and speed up the legacy job (+ reduced rate limit for Saucelabs).

PR Close #38435

tools/defaults.bzl

@@ -245,7 +245,10 @@ def karma_web_test_suite(name, **kwargs):
         runtime_deps = runtime_deps,
         bootstrap = bootstrap,
         deps = deps,
-        browsers = ["//dev-infra/browsers/firefox:firefox"],
+        browsers = [
+            "//dev-infra/browsers/chromium:chromium",
+            "//dev-infra/browsers/firefox:firefox",
+        ],
         data = data,
         tags = tags,
         **kwargs