Files.app: Removed the unused members of FileSelectionHandler.

After making the PreviewPanel class, some members of FileSelectionHandler are no
longer needed. This CL cleans them out.

BUG=284215
TEST=manually

Review URL: https://chromiumcodereview.appspot.com/24312004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224976 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224976