Clean up DriveAppRegistry (part 1 of 2).

- Removed unused fields from DriveAppInfo.
  It is easy to recover them later when needed.
- A bit of rewriting for FindPreferredIcon.

BUG=332014
R=hashimoto@chromium.org

Review URL: https://codereview.chromium.org/125163004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243465 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243465