IDL: initialize union member variables (to null/zero)

Since we now use non-null to represent union member presence, either by
wrapping with Nullable<> or using the implicit null value (for pointers
and strings,) we must make sure the locals we pass to the implementation
are all properly initialized to null.

For strings and RefPtr<>/OwnPtr<>, this requires no special care due to
default constructors taking care of it. For RawPtr<> (when used in
practice, i.e. mostly in Oilpan builds) we need to initialize explicitly.

This fixes Oilpan crashes introduced by this CL:
  https://codereview.chromium.org/466323002/

Review URL: https://codereview.chromium.org/474323002

git-svn-id: svn://svn.chromium.org/blink/trunk@180405 bbb929c8-8fbe-4397-9dbb-9b2b20218538