Add better resolution favicon for new tab topSites

[cezaraugusto]

Did you search for similar issues before submitting this one?
yes

Describe the issue you encountered:
Some websites (such as Youtube) doesn't display a good quality favicon by default, and appear pixelated on new tab's topSites.

Per Brad's request here (#3001) we should:

build high-res favicon collector mechanism, pulled from page header at domain root page

Expected behavior:
TopSites should have an aesthetically pleasing favicon.

My suggestion here:

I was thinking on getting images following Apple > MS conventions on links/meta icon declarations, then search for a given pattern for high res icons (maybe look for size attr, or maybe a given number). If all else fails we fallback to default *.ico images we already have. Like:

1. Search for Apple name convention
<link rel="apple-touch-icon" href="http://example.com/apple-touch-icon.png">

2. Search for MS name convention (tile pictures)
<meta name="msapplication-TileImage" content="http://example.com/mstile-72x72.png">

3. Search for any name convention for high-res icons (find for NxN occurencies or sizes attr)
<link type="image/png" rel="icon" href="http://example.com/something-144x144.png" sizes="144x144">

4. Fallback for default *.ico images

[cndouglas]

Related: #5658

[luixxiul]

I like the suggestion, still the background color should also be configurable to avoid the issue I mentioned on #5658

[bbondy]

Could I also ask that as part o this you re-use the favicon always for the sites that have them, that way they won't get overwritten at some point.