"Mixed Content" error for blocked requests not intercept-able

[CSchuetz7]

Steps to reproduce

Tell us about your environment:

• Puppeteer version:
  1.1.1
• Platform / OS version:
  ubuntu 16.04
• URLs (if applicable):
  https://a.df-srv.de/v2/view/lePQD7Tyr8SNUYizE5Yy/a.html
• Node.js version:
  9.7.1

What steps will reproduce the problem?
Load a page over https:// that has a http:// resource embeded in an <iframe> object.
(Example page: https://a.df-srv.de/v2/view/lePQD7Tyr8SNUYizE5Yy/a.html)

The debug console if devtools:true is enabled will show:
Mixed Content: The page at 'https://a.df-srv.de/v2/view/lePQD7Tyr8SNUYizE5Yy/a.html' was loaded over HTTPS, but requested an insecure resource 'http://ende.de/'. This request has been blocked; the content must be served over HTTPS.

However this message is not seen by puppeteer, even in the DEBUG=puppeteer:* logs there is no sign of the blocked request.

Please include code that reproduces the issue.

1. Load https://a.df-srv.de/v2/view/lePQD7Tyr8SNUYizE5Yy/a.html
2. See Mixed Content: The page at 'https://a.df-srv.de/v2/view/lePQD7Tyr8SNUYizE5Yy/a.html' was loaded over HTTPS, but requested an insecure resource 'http://ende.de/'. This request has been blocked; the content must be served over HTTPS.
3. Not intercept-able via requestfailed, Network.loadingFailed, Network.requestIntercepted, Network.requestWillBeSent or any other event.

What is the expected result?

Some way to catch the blocked Mixed Content log shown in the debug console.

What happens instead?

Nothing.

[MichaelErmer]

Related #1936

[aslushnikov]

These violations are reported via the Log domain that we don't enable yet. We need to enable domain and introduce a new page event that forwards messages
from Log domain to the clients.

I'll be happy to review a PR.

[aslushnikov]

Closing as dupe of #1939.