showContextMenu should use the host, not the user agent shadow elements.

This code does not expect to hit the inside of user agent shadow elements,
so we need to call setToShadowHostIfInUserAgentShadowRoot().

I'm not sure how to test this, it seems we have very little test coverage
for context menus?

BUG=393406

Review URL: https://codereview.chromium.org/393023005

git-svn-id: svn://svn.chromium.org/blink/trunk@178234 bbb929c8-8fbe-4397-9dbb-9b2b20218538