IDL compiler: [Clamp] for arguments

In practice used 2 places, for unsigned short and unsigned long.

One weird point (hence documented) is that here we use the literal IDL
type for a C++ variable.

I believe this is ok b/c |unsigned short| and |unsigned long| are legitimate
C++ types, even though we otherwise internally store them as |unsigned|,
but it does mean there's implicit type conversion going on when the
local variable is used later (since it's unsigned).

It might be more correct and safer to use the C++ type for the local variable,
only using the IDL type for the clampTo itself, and making the conversion
explicit here.

Docs:
http://www.w3.org/TR/WebIDL/#Clamp
http://www.chromium.org/blink/webidl/blink-idl-extended-attributes#TOC-Clamp-a-p-

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/53483004

git-svn-id: svn://svn.chromium.org/blink/trunk@161016 bbb929c8-8fbe-4397-9dbb-9b2b20218538