WebViewImpl's hit tests should use setToShadowHostIfInUserAgentShadowRoot.

This code in WebViewImpl never expects to end up inside a UA shadow root,
so call setToShadowHostIfInUserAgentShadowRoot() after doing each hit test.

Unfortunately there's no obvious way to test context menus right now.

I also added a bunch of FIXME's related to parentNode() traversals inside
WebViewImpl that are not ShadowDOM aware.

BUG=402026

Review URL: https://codereview.chromium.org/470803002

git-svn-id: svn://svn.chromium.org/blink/trunk@181071 bbb929c8-8fbe-4397-9dbb-9b2b20218538