Updating trunk VERSION from 3899.0 to 3900.0

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

Change-Id: Ic57b70cea298e07e84b097d8d85e36758df0e58e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1778990Reviewed-by: Chrome Release Bot (LUCI) <chrome-official-brancher@chops-service-accounts.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#692329}

chrome/VERSION

 MAJOR=78
 MINOR=0
-BUILD=3899
+BUILD=3900
 PATCH=0