Move all popup-related code in Source/web to the blink namespace

The majority of the work here was to disentangle ChromeClient from
these code paths: ChromeClient was being called from PopupContainer to
communicate with WebViewImpl. But PopupContainer is in Source/web, so
I've changed it to talk directly to WebViewImpl without going through
core. This avoids forward-declaring PopupContainer in ChromeClient.h.

Also did a bit of minor cleanup/dead code removal. Further cleanups
will happen later (e.g., handling the always-false second argument to
PopupContainer::popupOpened()).

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/154063004

git-svn-id: svn://svn.chromium.org/blink/trunk@166854 bbb929c8-8fbe-4397-9dbb-9b2b20218538