Updating trunk VERSION from 3720.0 to 3721.0

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

Change-Id: I6fbc60e4354ec4b4b05c6de41dce01f98022c0c0
Reviewed-on: https://chromium-review.googlesource.com/c/1492394Reviewed-by: chrome-release-bot@chromium.org <chrome-release-bot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636307}

chrome/VERSION

 MAJOR=74
 MINOR=0
-BUILD=3720
+BUILD=3721
 PATCH=0