[css-typed-om] Throw when setting multiple values on non list prop.

When setting multiple values on a non list valued property, we resolved
to throw a TypeError.

Spec:
https://github.com/w3c/css-houdini-drafts/issues/512

Bug: 785132
Change-Id: I2a1a06ffc3eb6db66e08e2537eeaa570feaff0d3
Reviewed-on: https://chromium-review.googlesource.com/826744Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524182}

third_party/WebKit/LayoutTests/typedcssom/the-stylepropertymap/set-expected.txt

@@ -3,6 +3,7 @@ PASS Setting a StylePropertyMap with an unsupported property name throws TypeErr
 PASS Setting a StylePropertyMap with an null property name throws TypeError
 PASS Setting a StylePropertyMap with an invalid CSSStyleValue throws TypeError
 PASS Setting a StylePropertyMap with an invalid String throws TypeError
+PASS Setting a non list-valued property with multiple arguments throws TypeError
 PASS Setting a property with CSSStyleValue or String updates its value
 PASS Setting a list-valued property with CSSStyleValue or String updates its values
 FAIL Setting a custom property with CSSStyleValue or String updates its value Failed to execute 'set' on 'StylePropertyMap': Invalid propertyName: --foo

third_party/WebKit/LayoutTests/typedcssom/the-stylepropertymap/set.html

@@ -22,6 +22,11 @@ for (const {property, values, desc} of gInvalidTestCases) {
   }, 'Setting a StylePropertyMap with ' + desc + ' throws TypeError');
 }
 
+test(() => {
+  let styleMap = newDivWithStyle().attributeStyleMap;
+  assert_throws(new TypeError(), () => styleMap.set('width', CSS.px(10), CSS.px(10)));
+}, 'Setting a non list-valued property with multiple arguments throws TypeError');
+
 test(() => {
   let styleMap = newDivWithStyle().attributeStyleMap;
 

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

@@ -97,8 +97,9 @@ void StylePropertyMap::set(const ExecutionContext* execution_context,
       SetProperty(property_id, result);
   } else {
     if (values.size() != 1) {
-      // FIXME: Is this actually the correct behaviour?
-      exception_state.ThrowTypeError("Not supported");
+      exception_state.ThrowTypeError(
+          "Cannot set " + property_name +
+          " with multiple values as it is not list-valued");
       return;
     }