CSSKeywordValue.value should never be empty string.

Currently CSSKeywordValue.value can be set to empty string if
empty string is assigned to that,instead it should 
throw a type error as per below spec.

Spec: https://github.com/w3c/css-houdini-drafts/issues/477

BUG: 776647

Change-Id: I4bb635a26e25dcb35d971bb2a7a1b7d3445a6aca
Reviewed-on: https://chromium-review.googlesource.com/745743
Commit-Queue: srirama chandra sekhar <srirama.m@samsung.com>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512818}

third_party/WebKit/LayoutTests/typedcssom/stylevalue-subclasses/cssKeywordValue-expected.txt

-This is a testharness.js-based test.
-PASS Constructing CSSKeywordValue with an empty string throws a TypeError
-FAIL Updating CSSKeywordValue.value with an empty string throws a TypeError assert_throws: function "() => result.value = value" did not throw
-PASS Constructing CSSKeywordValue with an object that serializes to empty string throws a TypeError
-FAIL Updating CSSKeywordValue.value with an object that serializes to empty string throws a TypeError assert_throws: function "() => result.value = value" did not throw
-PASS CSSKeywordValue can be constructed from a CSS keyword
-PASS CSSKeywordValue can be constructed from an unsupported CSS keyword
-PASS CSSKeywordValue can be constructed from a string containing multiple tokens
-PASS CSSKeywordValue can be constructed from a unicode string
-PASS CSSKeywordValue can be constructed from an object that serializes to a nonempty string
-Harness: the test ran to completion.
-

third_party/WebKit/Source/core/css/cssom/CSSKeywordValue.cpp

@@ -58,6 +58,16 @@ const String& CSSKeywordValue::value() const {
   return keyword_value_;
 }
 
+void CSSKeywordValue::setValue(const String& keyword,
+                               ExceptionState& exception_state) {
+  if (keyword.IsEmpty()) {
+    exception_state.ThrowTypeError(
+        "CSSKeywordValue does not support empty strings");
+    return;
+  }
+  keyword_value_ = keyword;
+}
+
 CSSValueID CSSKeywordValue::KeywordValueID() const {
   return CssValueKeywordID(keyword_value_);
 }

third_party/WebKit/Source/core/css/cssom/CSSKeywordValue.h

@@ -25,7 +25,7 @@ class CORE_EXPORT CSSKeywordValue final : public CSSStyleValue {
   StyleValueType GetType() const override { return kKeywordType; }
 
   const String& value() const;
-  void setValue(const String& keyword) { keyword_value_ = keyword; }
+  void setValue(const String& keyword, ExceptionState&);
   CSSValueID KeywordValueID() const;
 
   const CSSValue* ToCSSValue() const override;

third_party/WebKit/Source/core/css/cssom/CSSKeywordValue.idl

@@ -10,5 +10,5 @@
     Exposed(Window CSSTypedOM, PaintWorklet CSSTypedOM),
     RaisesException=Constructor
 ] interface CSSKeywordValue : CSSStyleValue {
-    attribute DOMString value;
+    [RaisesException=Setter] attribute DOMString value;
 };