Generate existing casting functions with DEFINE_TYPE_CASTS macro.

* Rename toPluginContainerImpl to toWebPluginContainerImpl for consistency.
* Remove unnecessary WebViewImpl->WebViewImpl casts.

BUG=309516

Review URL: https://codereview.chromium.org/105943008

git-svn-id: svn://svn.chromium.org/blink/trunk@164007 bbb929c8-8fbe-4397-9dbb-9b2b20218538