Updating trunk VERSION from 3865.0 to 3866.0 and incrementing major version to 78

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

Change-Id: I14e62dde98538aaed89091cba8be8f246de19ad8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720111Reviewed-by: chrome-release-bot@chromium.org <chrome-release-bot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681156}

chrome/MAJOR_BRANCH_DATE

-MAJOR_BRANCH_DATE=2019-05-31
+MAJOR_BRANCH_DATE=2019-07-26

chrome/VERSION

-MAJOR=77
+MAJOR=78
 MINOR=0
-BUILD=3865
+BUILD=3866
 PATCH=0