Why does Chrome on Android launch tabs or redirect existing tabs to spammy ads (spontaneously, it seems)? When using Chrome on my Nexus 6, I often get taken to a data URI that causes a modal that reads something like: 


  An embedded page at s3.amazonaws.com says:
  Message!
  Congratulations Grande Communications user, you have been chosen for a chance to get a new iPhone....
  Click OK to continue


There is no Cancel button. Why does Google allow developers to launch pop-ups with no cancel button? 

This doesn't occur from clicking on a link. Usually it happens while I'm on a tab, and suddenly another tab will spawn with the modal. I don't think it's caused by a visit to a dubious host, because it's happened when I've had only top sites (Amazon, ebay, etc.) open in the other tabs.

I have run Malwarebytes and a few other security tools and they all say my phone is secure and clean. 

What exactly is happening? On the standard Chrome browser, web extensions can rewrite URLs but Chrome for mobile doesn't have web extensions. Could an app intercept network calls from the browser? Could the sites I'm visiting just be serving malicious ads?
To build to what we know, I just saw this today on a stock Nexus6P (TMo) running the August update, only in 'ChromeBrowser'. Previously, it was happening to a roommate's unrooted Galaxy S4 (AT&amp;T), which now that I think about it, was running an even more current version of 'Chrome' than I was. No wifi, separate mobile networks. I updated in the past 24 hours and saw my first URI within an hour of googling this post and adding. The only websites I've used since the last boot have been Google, Wikipedia, Reddit, Amazon, and IMDB.

Tried clearing the system cache on both phones, uninstalling and reinstalling 'Chrome', running 'Malwarebytes' and 'Avast': still happens. Doesn't seem to be happening in 'Adblock Browser'.

If anyone can decipher the string that prompts mine, it looks like:

data:text/html;base64,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