Add toWebViewImpl(), and use it.

We can avoid static_cast by this change.
Unlike other toClassName functions, we can't add an assertion to confirm the
speicified WebView is actually a WebViewImpl. It's always true.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/22893059

git-svn-id: svn://svn.chromium.org/blink/trunk@156689 bbb929c8-8fbe-4397-9dbb-9b2b20218538