Updating trunk VERSION from 3939.0 to 3940.0

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

Change-Id: I1b06a4540c13b6e4628dec999fdda454f7a393a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1855220Reviewed-by: Chrome Release Bot (LUCI) <chrome-official-brancher@chops-service-accounts.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#704966}

chrome/VERSION

 MAJOR=79
 MINOR=0
-BUILD=3939
+BUILD=3940
 PATCH=0