LibWeb: Define width for -webkit-slider-runnable-track

Fixes #512

Userland/Libraries/LibWeb/CSS/Default.css

@@ -79,6 +79,7 @@ input[type=range]::-webkit-slider-runnable-track, input[type=range]::-webkit-sli
 }
 input[type=range]::-webkit-slider-runnable-track {
     height: 4px;
+    width: 100%;
     margin-top: 6px;
     background-color: AccentColor;
     border: 1px solid rgba(0, 0, 0, 0.5);