Cleanup RenderWidget.

- Remove frameView() and windowClipRect() from RenderWidget as it's dead code.
- Add explicit keyword to constructor that takes one argument.
- Remove unneeded 'using namespace std'.

BUG=none.

Review URL: https://codereview.chromium.org/106793003

git-svn-id: svn://svn.chromium.org/blink/trunk@163724 bbb929c8-8fbe-4397-9dbb-9b2b20218538