Use high resolution icons where possible for streamlined hosted app icons.

This adds a FaviconDownloader which downloads all icons 
when creating streamlined hosted apps from the current web
contents, providing higher resolution icons.

BUG=318607

Review URL: https://codereview.chromium.org/64853004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241462 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241462