Move external popup menus from WebViewClient to WebFrameClient, part 2/3.

BUG=304341
TEST=popup menus still work

Review URL: https://codereview.chromium.org/499463002

git-svn-id: svn://svn.chromium.org/blink/trunk@180785 bbb929c8-8fbe-4397-9dbb-9b2b20218538