Ensure chrome intent URIs, other unknown protocols embeded in pages, don't result in "Unknown protocol" pages

[mcomella]

Chrome intent URIs allow pages to open apps using URIs (e.g. <a href="intent://scan/#Intent;scheme=zxing;package=com.google.zxing.client.android;end"> Take a QR code </a>. A intuitive behavior for our developers is to show an error when seeing a protocol we can't handle. However, in my experience, some pages will try to open an intent Uri and fallback to a web page if the app is not available:

load(intentUri)
setTimeout(5000, {
  if (intentUriNotLoaded) {
    load(fallbackUrl)
  }
})

In the naive implementation, this will send the user to an "Unknown protocol" page. Even if we don't fully support the chrome intent URIs, we should ensure we don't do the wrong thing.

Note: later versions of a-c may get us this for free.

[mcomella]

My test page might be helpful here: http://mcomella.xyz/test/uri.html I think we should take no action when these links are clicked. If "Unknown protocol" shows up, we're probably doing the wrong thing but look at Chrome – we should follow their behavior (reasoning: even though I feel it's less correct, too many sites rely on Chrome's behavior so we must copy it).

[mcomella]

I confirmed that we have the "Unknown protocol behavior" both on master 4.7.1 and prior to 4.7 (i.e. this is not a regression). Fenix has the correct behavior so upgrading a-c is likely to address the issue.