Add UseCounters for the types of mixed content we currently don't block.

In order to evaluate when we can start dropping support for passive
mixed content, this patch adds counters to ResourceFetcher for the four
resource types we currently don't block: images, media, tracks, and raw.
This patch also drops the 'MixedContentFont' enum, as we now consider
that active content.

Review URL: https://codereview.chromium.org/312253003

git-svn-id: svn://svn.chromium.org/blink/trunk@175557 bbb929c8-8fbe-4397-9dbb-9b2b20218538