Count cross-origin property access.

It would be interesting to find out how often pages try to access
properties of cross-origin Windows, and equally interesting to determine
how many of those accesses are enabled by `window.open`.

R=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2881393002
Cr-Commit-Position: refs/heads/master@{#473112}
9 files changed