Expand tap highlight to allow multiple highlights for touch disambiguation.

Tap gestures may hit multiple targets, causing a disambiguation window to popup on Android. In addition to this, we would like all of the candidate targets highlighted rather than just the target which was scored highest.

This change allows multiple nodes to be highlighted simultaneously. The touch disambiguation is also tapped into to select nodes for highlighting.

BUG=286244

Review URL: https://chromiumcodereview.appspot.com/15663005

git-svn-id: svn://svn.chromium.org/blink/trunk@157947 bbb929c8-8fbe-4397-9dbb-9b2b20218538