Move Set() call in install_custom_signature to V8DOMConfiguration

To remove one FIXME. This CL adds installCallbackCustomSignature()
template function in V8DOMConfiguration, which selects an appropriate callback by checking whether the isolate is the main world and calls Set() on the given object.

BUG=345503

Review URL: https://codereview.chromium.org/352583003

git-svn-id: svn://svn.chromium.org/blink/trunk@176890 bbb929c8-8fbe-4397-9dbb-9b2b20218538