Updating trunk VERSION from 6390.0 to 6391.0

* This is an automated release commit.
* Do not revert without consulting chrome-pmo@google.com.
NOAUTOREVERT=true

Change-Id: I53a76516ed1925c791826de2dceb1f2f6961c5f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5409131
Bot-Commit: Chrome Release Bot (LUCI) <chrome-official-brancher@chops-service-accounts.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1280570}