Add use counters for Secure Context checks and sandboxed origins.

At https://github.com/w3c/webappsec-secure-contexts/issues/28, we're
discussing whether or not we can tighten the notion of "secure context"
to exclude sandboxed documents by default. This patch adds counters
to determine how marginal the breakage would be.

BUG=534788
TBR=isherman@chromium.org

Review-Url: https://codereview.chromium.org/2160533002
Cr-Commit-Position: refs/heads/master@{#405973}
3 files changed