Refactored loading of applications / extensions icons.
Added ImageLoader::LoadExtensionIconAsync() and ImageLoader::LoadExtensionIconDataURLAsync().
Moved all apps / extension loading code to ImageLoader.

BUG=297298,297301

Review URL: https://codereview.chromium.org/25050005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226823 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226823