Fix a regression regarding off-store extensions in metrics
This CL fixes a regression introduced by crrev.com/293777.
InstallVerifier::AllowedByEnterprisePolicy() are intended to query the policy system whether an extension are explicitly allowed by enterprise or not. But crrev.com/293777 changed the behavior into querying it's allowed to install or not, which in most cases, are true. But the ExtensionMetricsProvider considers every extension explicitly allowed by enterprise policy as from webstore, thus every off-store extensions are marked as from webstore in metrics.
Fix the regression by restoring into the original behavior.
BUG=423705
Review URL: https://codereview.chromium.org/
651253003
Cr-Commit-Position: refs/heads/master@{#299694}