Collect stats on how often we decline to use the plug-in type=&quotmime/blah&quot

      attribute and instead derive a MIME type based on the URL path.

BUG=75070
Review URL: http://codereview.chromium.org/7346042

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@92597 0039d316-1c4b-4281-b951-d872f2087c98