Move 'IDLExtendedAttributes.txt' filename to idl_validator.py

Currently 'IDLExtendedAttributes.txt' is hard-coded 4 places:
twice in .gyp, once in r-b-t (Tools/Scripts/webkitpy/bindings/main.py),
once in an error message
...and then passed down several levels to idl_validator.py where
it's actually used.
This is brittle (because r-b-t has a distant dependence on a path)
and verbose (we have yet another parameter to the compiler script and
constructors).

This moves the filename to idl_validator, reducing this to 2 places, not 4.
This means:
* r-b-t no longer depends on the file organization in bindings/,
* we can simplify the parameter lists, and
* the error message is up-to-date.

We still have the filename in .gyp (in the 'inputs'), which ensures it's
there and prevents build breakage.

R=haraken
BUG=341748

Review URL: https://codereview.chromium.org/218903002

git-svn-id: svn://svn.chromium.org/blink/trunk@170435 bbb929c8-8fbe-4397-9dbb-9b2b20218538