Change default content verification mode for Google Chrome builds

It's been configured to be set to 'enforce' mode via the server-side
experiment for quite a while, so it makes sense to have it turned on by
default in the code as well.

BUG=526873

Review URL: https://codereview.chromium.org/1307103003

Cr-Commit-Position: refs/heads/master@{#346552}
1 file changed