Add support for [Global] / [PrimaryGlobal] IDL extended attributes

Add support for [Global] / [PrimaryGlobal] IDL extended attributes as per the
latest Web IDL specification:
http://heycam.github.io/webidl/#Global

The [Global] and [PrimaryGlobal] extended attributes can be used to give a name
to one or more global interfaces, which can then be referenced by the [Exposed]
extended attribute (already supported).

One slight difference with the specification is that the values for [Global]
and [PrimaryGlobal] are '&'-separated instead of comma-separated. This is
because having comma-separated here would make IDL parsing a lot harder. This
is consistent with the [Exposed] IDL extended attribute.

There is no web exposed behavior change with this CL. It just makes our IDL
closer to the specification.

R=haraken@chromium.org, nbarth@chromium.org
BUG=369115

Review URL: https://codereview.chromium.org/261243002

git-svn-id: svn://svn.chromium.org/blink/trunk@173595 bbb929c8-8fbe-4397-9dbb-9b2b20218538