Fix null checks for layout in WebPagePopupImpl.

Like beginFrame, these are triggered in a callback by the compositor.
The page backing the WebPagePopupImpl may already be closed by the the
time the callback runs.

Other cleanups:
- Since all callers have to check that Page is not null before
  forwarding the call to PageWidgetDelegate anyway, pass Page& by
  reference.
- Since animate(), beginFrame(), and layout() don't check that the
  supplied LocalFrame is not null before using it, pass LocalFrame& by
  reference as well.
- Finally, PageWidgetDelegate::handleInputEvent() wasn't using the Page
  parameter, so just remove it.

BUG=420118

Review URL: https://codereview.chromium.org/626853004

git-svn-id: svn://svn.chromium.org/blink/trunk@183294 bbb929c8-8fbe-4397-9dbb-9b2b20218538