Measure usage of generated documents (image/media/plugin/etc)

As discussed on blink-dev:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/u79ubJvsTbI/c_kSGKrEAgAJ

This *InFrame counters are intended as an estimate of cases where a
script in the containing document could look at the generated DOM.

BUG=597380
R=esprehn@chromium.org,pdr@chromium.org
TBR=isherman@chromium.org

Review URL: https://codereview.chromium.org/1866343002

Cr-Commit-Position: refs/heads/master@{#388723}
9 files changed