Report precache metrics when fetches complete.

BUG=306185, 309216

Review URL: https://codereview.chromium.org/106553005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241670 0039d316-1c4b-4281-b951-d872f2087c98
Pass untrusted (plugin->host) messages through NaClMessageScanner.

This will be used for auditing fileio / filesystem messages to
verify the plugin isn't cheating on quota.

BUG=194304

Review URL: https://codereview.chromium.org/116543003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241671 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241670
SVN changes up to revision 241671