Remove PlatformThemeChromiumDefault as it is dead code

There are only two calls to it in WebViewImpl but those has no effect.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/17279002

git-svn-id: svn://svn.chromium.org/blink/trunk@159988 bbb929c8-8fbe-4397-9dbb-9b2b20218538