Allow touch adjustment to return Shadow DOM node.

Touch adjustment currently considers a shadow DOM node when finding the best
clickable one but then walks up the tree to the shadow host and returns it.
Make it return the shadow node instead so that we don't have to hit test again
when processing a gesture tap.

BUG=391524

Review URL: https://codereview.chromium.org/370843002

git-svn-id: svn://svn.chromium.org/blink/trunk@177885 bbb929c8-8fbe-4397-9dbb-9b2b20218538