IDL: Support 'stringifier' keyword

Usable in one of these forms:

  stringifier;
  stringifier DOMString ();
  stringifier DOMString name();
  stringifier attribute DOMString name;

All cause a toString() method to be defined on the interface automatically.
The first two are equivalent, and call CPPClass::toString() to retrieve the
string value. The latter two use CPPClass::name() to retrieve the string
value and also declare a regular attribute and operation, respectively.

Changed all simple "DOMString toString()" occurrences in IDL files to use
"stringifier" instead.

BUG=306606

Review URL: https://codereview.chromium.org/345393004

git-svn-id: svn://svn.chromium.org/blink/trunk@176833 bbb929c8-8fbe-4397-9dbb-9b2b20218538