Teach the IDL compiler about TreatNullAs=EmptyString

Source/bindings/scripts/code_generator_v8.pm

Index: Source/bindings/scripts/code_generator_v8.pm
diff --git a/Source/bindings/scripts/code_generator_v8.pm b/Source/bindings/scripts/code_generator_v8.pm
index 949a2262119f32bb250e87c8a1dc740f64289c6e..79272de3041e0b001bfe13f596aa894e9151df57 100644
--- a/Source/bindings/scripts/code_generator_v8.pm
+++ b/Source/bindings/scripts/code_generator_v8.pm
@@ -676,12 +676,13 @@ sub GetV8StringResourceMode
 {
     my $extendedAttributes = shift;
 
-    # Blink uses the non-standard identifier NullString instead of Web IDL
-    # standard EmptyString, in [TreatNullAs=NullString] and [TreatUndefinedAs=NullString],
-    # and does not support [TreatUndefinedAs=Null] or [TreatUndefinedAs=Missing]
+    # In addition to [TreatNullAs=EmptyString], Blink also supports the non-standard
+    # [TreatNullAs=NullString] and formerly-standard [TreatUndefinedAs=NullString].
     # https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-TreatNullAs-a-p-TreatUndefinedAs-a-p-
     my $mode = "";
-    if (($extendedAttributes->{"TreatNullAs"} and $extendedAttributes->{"TreatNullAs"} eq "NullString") and ($extendedAttributes->{"TreatUndefinedAs"} and $extendedAttributes->{"TreatUndefinedAs"} eq "NullString")) {
+    if ($extendedAttributes->{"TreatNullAs"} and $extendedAttributes->{"TreatNullAs"} eq "EmptyString") {
+        $mode = "TreatNullAsEmptyString";
+    } elsif (($extendedAttributes->{"TreatNullAs"} and $extendedAttributes->{"TreatNullAs"} eq "NullString") and ($extendedAttributes->{"TreatUndefinedAs"} and $extendedAttributes->{"TreatUndefinedAs"} eq "NullString")) {
         $mode = "WithUndefinedOrNullCheck";
     } elsif ($extendedAttributes->{"TreatNullAs"} and $extendedAttributes->{"TreatNullAs"} eq "NullString") {
         $mode = "WithNullCheck";