IDL compiler: Refactor to move interface-wide contents to v8_interface.py/interface.cpp

This CL simplifies v8_attributes.py and v8_methods.py to only generate
template contents for a single attribute or single method, moving the
interface-wide contents to v8_interface.py, in some cases inlining the
simple logic into interface.cpp.

This is more correct, as these are used at the overall interface level,
and better balances the code:
* it simplifies the already complex v8_attributes
* ...and soon to be complex v8_methods.py, and
* it makes v8_interface.py more than a thin wrapper.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/43343002

git-svn-id: svn://svn.chromium.org/blink/trunk@160687 bbb929c8-8fbe-4397-9dbb-9b2b20218538