Add initial support for [Exposed=xx] extended IDL attribute

Add initial support for [Exposed=xx] extended IDL attribute:
http://heycam.github.io/webidl/#Exposed

This is adapted from our existing Blink-specific [GlobalContext=xx] IDL
extended attribute as the functionality is identical (select on which
global interface the interface is exposed).

There are 2 remaining issues that I am planning to address later on:
- The mapping between global identifiers and global IDL interfaces is currently
  manual but is supposed to rely on [Global] / [PrimaryGlobal] attributes
  instead, which we do not support yet.
- The values are still separated by '&' instead of ',' as per Web IDL
  specification. Having comma-separated values here confuses the parser as
  extended IDL attributes are already comma-separated. I was not able to add
  support for this in the Blink IDL lexer yet.
(Nils: using commas to separate makes parsing *very* hard,
and I don't think it's worth supporting.)

There is no web-exposed behavior change. This change just makes our Blink IDL
more standard.

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

Review URL: https://codereview.chromium.org/265773010

git-svn-id: svn://svn.chromium.org/blink/trunk@173168 bbb929c8-8fbe-4397-9dbb-9b2b20218538