IDL: Add V8DOMConfiguratin::installMethod()

We should use the same logic to install methods for both |install_methods|
and |install_per_context_methods| blocks because they should do the same thing
except for the type of the target can be different(object or V8's template).
This CL also removes installMethodCustomSignature() because the caller can use
installMethod() instead.

Review URL: https://codereview.chromium.org/447683002

git-svn-id: svn://svn.chromium.org/blink/trunk@179607 bbb929c8-8fbe-4397-9dbb-9b2b20218538