Make Gesture Tap be aware of Shadow DOM.

There are several places where we should use NodeRenderingTraversal::parent rather than plain node->parentNode() in WebViewImpl.
Unless that, we can't handle the case where a distributed node is tapped.

This patch also fixed the crash which a newly added layout test, gesture-tapHighlight-shadow-tree.html, has revealed.

In LinkHighlight::computeQuads, we can't use node->childNode() to retrieve a child render object here
because a render tree is constructed from a composed tree and we should retrieve a render object of a distributed node here.

BUG=399522
TEST=gesture-tapHighlight-shadow-tree.html

Review URL: https://codereview.chromium.org/448593002

git-svn-id: svn://svn.chromium.org/blink/trunk@180643 bbb929c8-8fbe-4397-9dbb-9b2b20218538