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Prior to this patch, desktop favicon downloading would only stop when
a truly ideal icon is found, which means a favicon with perfect bitmaps
sizes for all supported screen densities (often a .ico file with a
16x16 AND a 32x32 bitmap). This often leaded to all icons being
downloaded, even for cases where it's obviously unnecessary.

The patch adopts the logic we currently use on mobile within
FaviconHandler::UpdateFaviconCandidate() in order to stop processing
favicon candidates if there is little hope that the next ones will be
any better.

This requires some smartness for favicon candidates without an explicit
'sizes' attribute, which would score very low prior to this patch and
hence would rarely be processed (if other candidates exist). Hence,
we special-case them for desktop and score them highest during initial
sorting, which mimics the behavior prior to this patch and guarantees
they will be processed unless an ideal favicon is found before.

It's rather trivial to rule out regressions for the following common
scenarios:
A. Mobile platforms: no behavioral changes as score stays zero for
   candidates without explicit 'sizes' attribute.
B. A single favicon is listed by the page.
C. Multiple favicons are listed and none have a 'sizes' attribute
   (sorting does nothing and all are processed unless an ideal one is
   found).
D. Multiple favicons are listed and they all have an accurate 'sizes'
   attribute (scenario improved in this patch).

The more complex scenario is when a web page lists multiple favicons
and only some have a 'sizes' attribute. In this case, there is some
minor risk for regressions given that the patch scores highest the
favicons without a 'sizes' attribute, so they will be downloaded and
processed earlier.

Bug: 895175,705900,672076
Change-Id: I8216652ede25f55332cacc54d3e0a806fd8b7bc3
Reviewed-on: https://chromium-review.googlesource.com/c/1323551
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610800}