IDL: Support empty sequence default value

The spec[1] says that we can use '[]' to specify a default value
which represents an empty sequence value. Note that this CL just
makes idl_parser recognize an empty sequence value. Actual work
should be done by clients (e.g. Blink's code generator).

[1] http://heycam.github.io/webidl/#dfn-optional-argument-default-value

BUG=450385
TEST=tools/idl_parser/run_tests.py

Review URL: https://codereview.chromium.org/865453002

Cr-Commit-Position: refs/heads/master@{#312380}

2 files changed, 23 insertions, 2 deletions

diff --git a/tools/idl_parser/idl_parser.py b/tools/idl_parser/idl_parser.py
index af0bd05..0fa9b61 100755
--- a/tools/idl_parser/idl_parser.py
+++ b/tools/idl_parser/idl_parser.py
@@ -324,8 +324,12 @@ class IDLParser(object):
   # [17]
   def p_DefaultValue(self, p):
     """DefaultValue : ConstValue
-                    | string"""
-    if type(p[1]) == str:
+                    | string
+                    | '[' ']'"""
+    if len(p) == 3:
+      p[0] = ListFromConcat(self.BuildAttribute('TYPE', 'sequence'),
+                            self.BuildAttribute('VALUE', '[]'))
+    elif type(p[1]) == str:
       p[0] = ListFromConcat(self.BuildAttribute('TYPE', 'DOMString'),
                             self.BuildAttribute('NAME', p[1]))
     else:
diff --git a/tools/idl_parser/test_parser/interface_web.idl b/tools/idl_parser/test_parser/interface_web.idl
index e42075b..cff2cd0 100644
--- a/tools/idl_parser/test_parser/interface_web.idl
+++ b/tools/idl_parser/test_parser/interface_web.idl
@@ -75,6 +75,23 @@ interface MyIFaceBig {
 };
 
 /* TREE
+ *Interface(MyIfaceEmptySequenceDefalutValue)
+ *  Operation(foo)
+ *    Arguments()
+ *      Argument(arg)
+ *        Type()
+ *          Sequence()
+ *            Type()
+ *              PrimitiveType(DOMString)
+ *        Default()
+ *    Type()
+ *      PrimitiveType(void)
+ */
+interface MyIfaceEmptySequenceDefalutValue {
+  void foo(optional sequence<DOMString> arg = []);
+};
+
+/* TREE
  *Interface(MyIFaceBig2)
  *  Const(nullValue)
  *    PrimitiveType(DOMString)