Allow karma browsers parameter to be set from bazel

When running karma tests through bazel, ChromeHeadless is the default
browser. Currently there is no option to change the browser without
editing polygerrit-ui/karma.conf.js. This change makes it easier for
CQ to use ChromiumHeadless.

Bug: 1196790
Change-Id: I4328120f33a630b33f9345ce908684fdbeef88c8
diff --git a/web/karma_test.sh b/web/karma_test.sh
index e21aa13..1491768 100755
--- a/web/karma_test.sh
+++ b/web/karma_test.sh
@@ -3,4 +3,5 @@
 set -euo pipefail
 ./$1 start $2 --single-run \
   --root 'plugins/landingwidget/web/_bazel_ts_out_tests/' \
-  --test-files '*_test.js'
+  --test-files '*_test.js' \
+  --browsers ${3:-ChromeHeadless}