idl_parser: Support annotated types in IDL files

This CL's intention is to adapt IDL files to the change of WebIDL's
specifications, to support annotated types.

Before this CL, sometimes types which are applicable to types ([Clamp],
 [EnforceRange], [TreatNullAs]) applied to non-types, even when
 specifications say otherwise.
After this CL, however, such types got to apply to types.

Actually there are still types in specifications which are applicable to types but apply to
non-types, and we'll report those bugs to the specifications.

Bug: 714866
Change-Id: I1c8a234f6ffc0c8b02681c46b32867d9f30f2097
Reviewed-on: https://chromium-review.googlesource.com/674473
Commit-Queue: Lisa Suzuki 
Reviewed-by: Yuki Shiino 
Reviewed-by: Hitoshi Yoshida 
Reviewed-by: Kentaro Hara 
Reviewed-by: Kenichi Ishibashi 
Cr-Commit-Position: refs/heads/master@{#503327}

Author: Lisa Suzuki

interfaces/dom.idl

@@ -420,7 +420,7 @@ interface Attr : Node {
 
 [Exposed=Window]
 interface CharacterData : Node {
-  [TreatNullAs=EmptyString] attribute DOMString data;
+  attribute [TreatNullAs=EmptyString] DOMString data;
   readonly attribute unsigned long length;
   DOMString substringData(unsigned long offset, unsigned long count);
   void appendData(DOMString data);