IDL: Add support for [TreatNullAs=EmptyString]

[TreatNullAs=NullString] was already supported, and still is, but is
non-standard.  [TreatNullAs=EmptyString] is what Web IDL has and what is
used in specifications to describe required behavior.

There are cases where web-exposed behavior relating to handling of null
is wrong today, and where using [TreatNullAs=EmptyString] instead would
be the natural fix.

BUG=391194

Review URL: https://codereview.chromium.org/368313005

git-svn-id: svn://svn.chromium.org/blink/trunk@177561 bbb929c8-8fbe-4397-9dbb-9b2b20218538