[Files.app] Call the parent constructor in the constructor of DirectoryTreeItem

DirectoryTreeItem didn't use the constructor of the parent class so it may be incompatible with the original tree.js and can't get the advantage of the tree.js (eg. a11y support). This patch has it call the constructor.

BUG=295899
TEST=manually tested
R=hirono@chromium.org

Review URL: https://codereview.chromium.org/140393004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247373 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247373