IDL compiler: Special operations with identifiers

Add support for special operations with identifiers (method name), like:
getter DOMString item(unsigned long index);

This is identical to specifying a normal operation + and anonymous special operation,
and then telling the special op to use the specified normal op:
DOMString item(unsigned long index);
[ImplementedAs=item] getter DOMString (unsigned long index);

I.e., the CG generates a normal op, and the (interface-wide) special op.
...so the code change is just to use the name, if present, and remove the
"anonymous" from all function names/template block names.

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/149313002

git-svn-id: svn://svn.chromium.org/blink/trunk@165995 bbb929c8-8fbe-4397-9dbb-9b2b20218538