Files.app: Extract the list specific behavior from DragSelector.

Previously the DragSelector class has the file list specific code. This CL
extracts the specific behavior into the method of the file list and remove the
dependency from DragSelector.

BUG=247278
TEST=manually
R=yoshiki@chromium.org

Review URL: https://codereview.chromium.org/26574006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228152 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228152