Add counters for various ways of loading scripts with bad mimetypes

      Unchecked use of proxies could enable cross-origin reading of some CSV
files which contain sensitive non-numerical information. Tightening
down mimetype checking could provide a mitigating strategy. This patch
adds UseCounters to see how often scripts are used with different
mimetypes to determine what could be prohibited.

BUG=chromium:399951
R=adamk

Review URL: https://codereview.chromium.org/1413193010

Cr-Commit-Position: refs/heads/master@{#364013}