Move modal dialogs from WebViewClient to WebFrameClient, part 2/3.

BUG=304341
TEST=modal dialogs still work

Review URL: https://codereview.chromium.org/226413004

git-svn-id: svn://svn.chromium.org/blink/trunk@171180 bbb929c8-8fbe-4397-9dbb-9b2b20218538