Updating trunk VERSION from 3945.0 to 3946.0 and incrementing major version to 80

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

Change-Id: I10fa95860e79cb9b7690cdde94bdc4dc39206173
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868330Reviewed-by: Chrome Release Bot (LUCI) <chrome-official-brancher@chops-service-accounts.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#707232}

chrome/MAJOR_BRANCH_DATE

-MAJOR_BRANCH_DATE=2019-07-26
+MAJOR_BRANCH_DATE=2019-10-18

chrome/VERSION

-MAJOR=79
+MAJOR=80
 MINOR=0
-BUILD=3945
+BUILD=3946
 PATCH=0