Remove forward declared NSMenu/NSResponder from ChromeClient.h

This was added in http://trac.webkit.org/changeset/30740

Neither symbol is used in ChromeClient.h.

core/rendering/RenderThemeChromiumMac.h uses NSMenu, but appears to
get the declaration/definition from elsewhere, together with a few
other NS* symbols.

Review URL: https://codereview.chromium.org/352783003

git-svn-id: svn://svn.chromium.org/blink/trunk@176923 bbb929c8-8fbe-4397-9dbb-9b2b20218538