Allow Installation with a policy-installed cert

Installation requires that the security level of the page be SECURE or
EV_SECURE, but this omitted the equivalent state on ChromeOS with a
policy-installed certificate. This CL ensures that all valid HTTPS
states permit installation.

Bug: 794256
Change-Id: I8ac7f2e6a7ca215197a32ec5ee534410d20d82b0
Reviewed-on: https://chromium-review.googlesource.com/823152Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Eric Lawrence <elawrence@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523592}

chrome/browser/installable/installable_manager.cc

@@ -159,8 +159,7 @@ bool InstallableManager::IsContentSecure(content::WebContents* web_contents) {
   security_state::SecurityInfo security_info;
   SecurityStateTabHelper::FromWebContents(web_contents)
       ->GetSecurityInfo(&security_info);
-  return security_info.security_level == security_state::SECURE ||
-         security_info.security_level == security_state::EV_SECURE;
+  return security_state::IsSslCertificateValid(security_info.security_level);
 }
 
 // static