Remove the support of DoNotCrossUpperBoundary mode from ComposedTreeWalker.

FocusController has been the only user of *DoNotCrossUpperBoundary* mode. I have updated FocusController so that it can traverse nodes based on a tree of trees without using ComposedTreeWalker recently.

Now we can remove DoNotCrossUpperBoundary mode from ComposedTreeWalker.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/23865003

git-svn-id: svn://svn.chromium.org/blink/trunk@157274 bbb929c8-8fbe-4397-9dbb-9b2b20218538