Fix FaviconHandler when it has multiple icon URLs and one of them is invalid

Fix FaviconHandler when it has multiple icon URLs and one of them is invalid

This CL prevents FaviconHandler from stopping iteration over a page's icon URLs
if a download for one of the icon URLs previously returned a 404. In particular,
this CL makes FaviconHandler::ScheduleDownload() call the download callback
when FaviconDriver::StartDownload() does not start a download. This is important
because the download callback is responsible for initiating the fetch for the
data for the next favicon candidate.

Sample scenario:
  Page HTML snippet:
  <link rel="icon" href="www.google.com/nothingtoseehere.png" />
  <link rel="icon" href="www.google.com/favicon.ico" />

  A previous download for "www.google.com/nothingtoseehere.png" returned a 404
  (e.g. the user previously visited a different page which also makes use of
  www.google.com/nothingtoseehere.png)

  This CL makes FaviconHandler fetch data for "www.google.com/favicon.ico"
  despite the download for the first favicon candidate
  ("www.google.com/nothingtoseehere.png") having previously failed.

BUG=474429
TEST=FaviconHandlerTest.MultipleFavicons404,
     FaviconHandlerTest.MultipleFaviconsAll404

Review URL: https://codereview.chromium.org/1079523005

Cr-Commit-Position: refs/heads/master@{#327339}