weblayer, skew tests: Add service ID to _KNOWN_ROBOTS in PRESUBMIT.py

This will allow CL's uploaded by the chrome-weblayer-builder@chops-service-accounts.iam.gserviceaccount.com
to bypass the AUTHORS file check so that it can submit CLs. The
service ID is uploading CL's from the following infra.cron job,
https://ci.chromium.org/p/infra/builders/cron/create-weblayer-skew-tests.
The recipe checks Chromiumdash for new Anroid releases in beta and
creates skew tests for Weblayer for each release. The recipe makes
changes to src/testing/buildbot/variants.pyl in order to create skew
tests. Entries like WEBLAYER_CLIENT_SKEW_TESTS_NTH_MINUS_TWO_MILESTONE
in variants.pyl are used to generate skew test suites in the buildbot
json files.

Bug: 1137911,1041619,1099017
Change-Id: I6b1928d123197defba2a8eb4beb3cff4e866019f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2469400
Commit-Queue: Rakib Hasan <rmhasan@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817069}

PRESUBMIT.py

@@ -1394,7 +1394,7 @@ _KNOWN_ROBOTS = set(
   ) | set('%s@developer.gserviceaccount.com' % s for s in ('3su6n15k.default',)
   ) | set('%s@chops-service-accounts.iam.gserviceaccount.com' % s
           for s in ('bling-autoroll-builder', 'v8-ci-autoroll-builder',
-                    'wpt-autoroller',)
+                    'wpt-autoroller', 'chrome-weblayer-builder')
   ) | set('%s@skia-public.iam.gserviceaccount.com' % s
           for s in ('chromium-autoroll', 'chromium-release-autoroll')
   ) | set('%s@skia-corp.google.com.iam.gserviceaccount.com' % s