Drop support for [TreatUndefinedAs=NullString]

third_party/WebKit/Source/bindings/IDLExtendedAttributes.md

 # Blink IDL Extended Attributes
 
 [TOC]
 
 ## Introduction
 
 The main interest in extended attributes are their _semantics_: Blink implements many more extended attributes than the Web IDL standard, to specify various behavior.
 
 The authoritative list of allowed extended attributes and values is [bindings/IDLExtendedAttributes.txt](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/bindings/IDLExtendedAttributes.txt). This is complete but not necessarily precise (there may be unused extended attributes or values), since validation is run on build, but coverage isn't checked.
 
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 Summary: Signals that a `readonly` attribute that returns an object type always returns the same object.
 
 This attribute has no effect on code generation and should simply be used in Blink IDL files if the specification uses it. If you want the binding layer to cache the resulting object, use `[SaveSameObject]`.
 
 ### [SecureContext] _(a, i, m)_
 
 Standard: [SecureContext](https://heycam.github.io/webidl/#SecureContext)
 
 Summary: Interfaces and interface members with a `SecureContext` attribute are exposed only inside ["Secure Contexts"](https://w3c.github.io/webappsec-secure-contexts/).
 
-### [TreatNullAs] _(a,p)_, [TreatUndefinedAs] _(a,p)_
+### [TreatNullAs] _(a,p)_
 
-Standard: [TreatNullAs](http://heycam.github.io/webidl/#TreatNullAs)
+Standard: [TreatNullAs](https://heycam.github.io/webidl/#TreatNullAs)
 
-`[TreatUndefinedAs]` has been been removed from the spec.
+Summary: `[TreatNullAs=EmptyString]` indicates that a JavaScript null is converted to `""` instead of `"null"`.
 
-Summary: They control the behavior when a JavaScript null or undefined is passed to a DOMString attribute or parameter.
-
-Implementation: **Non-standard:** Web IDL specifies the EmptyString identifier for both these extended attributes, and `Null` and `Missing` for `TreatUndefinedAs`. Blink uses the `NullString` identifier instead of `EmptyString` which yields a Blink null string, corresponding to JavaScript `null`, for which both `String::IsEmpty()` and `String::IsNull()` return true, instead of the empty string `""`, which is empty but not `null`. This is for performance reasons; see [Strings in Blink](https://docs.google.com/a/google.com/document/d/1kOCUlJdh2WJMJGDf-WoEQhmnjKLaOYRbiHz5TiGJl14/edit) for reference. It also does not implement `TreatUndefinedAs=Null` or `TreatUndefinedAs=Missing`.
-
-Further, both extended attributes are both overloaded for indexed setters and named setters, in which case they take a method name (?), though this usage is rare.
-
-Usage: The possible usage is `[TreatNullAs=NullString]` or `[TreatNullAs=NullString, TreatUndefinedAs=NullString]`. They can be specified on DOMString attributes or DOMString parameters only:
+Usage: Can be specified on DOMString attributes or DOMString parameters only:
 
 ```webidl
-[TreatNullAs=NullString] attribute DOMString str;
-void func([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString str);
+[TreatNullAs=EmptyString] attribute DOMString str;
+void func([TreatNullAs=Emptytring] DOMString str);
 ```
 
-`[TreatNullAs=NullString]` indicates that if a JavaScript null is passed to the attribute or parameter, then it is converted to a Blink null string, for which both String::IsEmpty() and String::IsNull() will return true. Without `[TreatNullAs=NullString]`, a JavaScript null is converted to a Blink string "null".
-`[TreatNullAs=NullString]` in Blink corresponds to `[TreatNullAs=EmptyString]` in the Web IDL spec. Unless the spec specifies `[TreatNullAs=EmptyString]`, you should not specify `[TreatNullAs=NullString]` in Blink.
+Implementation: Given `[TreatNullAs=EmptyString]`, a JavaScript null is converted to a Blink empty string, for which `String::IsEmpty()` returns true, but `String::IsNull()` return false.
 
-`[TreatUndefinedAs=NullString]` indicates that if a JavaScript undefined is passed to the attribute or parameter, then it is converted to a Blink null string, for which both String::IsEmpty() and String::IsNull() will return true. Without `[TreatUndefinedAs=NullString]`, a JavaScript undefined is converted to a Blink string "undefined".
-
-`[TreatUndefinedAs=NullString]` in Blink corresponds to `[TreatUndefinedAs=EmptyString]` in the Web IDL spec. Unless the spec specifies `[TreatUndefinedAs=EmptyString]`, you should not specify `[TreatUndefinedAs=NullString]` in Blink.
+**Non-standard:** Blink also supports the `NullString` identifier. Given `[TreatNullAs=NullString]`, a JavaScript null is converted to a Blink null string, for which both `String::IsEmpty()` and `String::IsNull()` return true. This is for performance reasons; see [Strings in Blink](https://docs.google.com/a/google.com/document/d/1kOCUlJdh2WJMJGDf-WoEQhmnjKLaOYRbiHz5TiGJl14/edit) for reference. Unless the spec specifies `[TreatNullAs=EmptyString]`, you should not specify `[TreatNullAs=NullString]` in Blink. Care must be taken to not treat null and empty strings differently, as they would be indistinguishable when using `[TreatNullAs=EmptyString]`.
 
 ### [Unforgeable] _(m,a)_
 
 Standard: [Unforgeable](http://heycam.github.io/webidl/#Unforgeable)
 
 Summary: Makes interface members unconfigurable and also controls where the member is defined.
 
 Usage: Can be specified on methods, attributes or interfaces:
 
 ```webidl
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