Enables MAYBE_BOOL flags for when you want to only do something if the flag
was explicitly set to true or false. Also cleans up JSArguments struct.

BUG=None
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23513062

Patch from Ross McIlroy <rmcilroy@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16774 ce2b1a6d-e550-0410-aec6-3dcde31c8c00