Add ByteString support to IDL bindings

ByteString is like DOMString, but should throw TypeError 
during the V8->C++ hop if script tries to set a string with a 
code unit greater than 0x00FF. No enforcement is done on the 
C++->V8 side.

R=nbarth@chromium.org
BUG=347426

Review URL: https://codereview.chromium.org/309553002

git-svn-id: svn://svn.chromium.org/blink/trunk@175995 bbb929c8-8fbe-4397-9dbb-9b2b20218538