Provide a prefixed appearance CSS rule for reference testing in Chrome

In `rasterizeAnnotationLayer` we load the source CSS files directly, so
these are not processed by Autoprefixer. Since the prefixed rules have
now been removed from the source CSS files, we must manually provide one
prefixed rule that Chrome needs in the overrides CSS file for checkbox
and radio button rendering to work in the reference tests.

test/annotation_layer_builder_overrides.css

@@ -19,6 +19,11 @@
   position: absolute;
 }
 
+.annotationLayer .buttonWidgetAnnotation.checkBox input,
+.annotationLayer .buttonWidgetAnnotation.radioButton input {
+  -webkit-appearance: none;
+}
+
 .annotationLayer .linkAnnotation > a,
 .annotationLayer .buttonWidgetAnnotation.pushButton > a {
   opacity: 0.2;