Mitigate the pushState IPC storm DoS.

This is nothing like what the ultimate, true solution should look like. It does
seem to work for the narrow problem of calling history.pushState in a tight
loop, however.

BUG=394296

Review-Url: https://codereview.chromium.org/2972073002
Cr-Commit-Position: refs/heads/master@{#485498}
3 files changed