IDL: Support argument default values

This also changes the code generated for existing null default values,
by dropping use of argumentOrNull() (which then becomes unused and is
removed) and setting the argument's local variable explicitly by
assigning "nullptr" to it.

The use of argumentOrNull() in V8HistoryCustom.cpp was redundant;
reading info[N] directly yields an undefined value, which is converted
to a null string by V8StringResource<WithUndefinedOrNullCheck>.

BUG=258153

Review URL: https://codereview.chromium.org/335113002

git-svn-id: svn://svn.chromium.org/blink/trunk@176861 bbb929c8-8fbe-4397-9dbb-9b2b20218538