Fix type confusion in BuilderConverter::applyValueContent.

https://bugs.webkit.org/show_bug.cgi?id=255955.
rdar://108499561.

Reviewed by Antti Koivisto.

This change fixes applyValueContent so that it can deal with single
values instead of expecting a list of values towards the end.

* LayoutTests/fast/css/style-builder-apply-value-content-type-confusion-expected.txt: Added.
* LayoutTests/fast/css/style-builder-apply-value-content-type-confusion.html: Added.
* Source/WebCore/style/StyleBuilderCustom.h:
  (WebCore::Style::BuilderCustom::applyValueContent):

Originally-landed-as: 259548.730@safari-7615-branch (c123784). rdar://113168576
Canonical link: https://commits.webkit.org/266585@main

LayoutTests/fast/css/style-builder-apply-value-content-type-confusion-expected.txt

@@ -0,0 +1,2 @@
+
+PASS if no crash.

LayoutTests/fast/css/style-builder-apply-value-content-type-confusion.html

@@ -0,0 +1,9 @@
+<script>
+  onload = () => {
+    if (window.testRunner)
+        testRunner.dumpAsText();
+    d.attributeStyleMap.set('content', 'url(A)');
+  };
+</script>
+<div id="d"></div>
+<p>PASS if no crash.</p>