Updating trunk VERSION from 4020.0 to 4021.0

# This is an automated release commit.
# Do not revert without consulting chrome-pmo@google.com.
NOAUTOREVERT=true
TBR=mmoss@chromium.org

Change-Id: I92de761095b0e7b7fa0e65a0e65df14df13560a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1988740Reviewed-by: Chrome Release Bot (LUCI) <chrome-official-brancher@chops-service-accounts.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#728782}

chrome/VERSION

 MAJOR=81
 MINOR=0
-BUILD=4020
+BUILD=4021
 PATCH=0