Add certificate error handling to devtools.

This is necessary as headless chrome cannot show a UI warning for SSL
certificate errors. Instead, we can expose the errors as DevTools events
and control the action to take through a DevTools command.

BUG=659662

Review-Url: https://codereview.chromium.org/2639203003
Cr-Commit-Position: refs/heads/master@{#458673}
10 files changed