WebUI: cr-slider, update obsolete Shadow DOM v0 CSS var fallback syntax

Bug: 908003
Change-Id: Iada2d816073d4f38cd0438838548d10f94d5b7db
Reviewed-on: https://chromium-review.googlesource.com/c/1352944
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611924}

ui/webui/resources/cr_elements/cr_slider/cr_slider.html

@@ -44,7 +44,7 @@
       }
 
       #bar {
-        border-top-color: var(--cr-slider-active-color, --google-blue-600);
+        border-top-color: var(--cr-slider-active-color, var(--google-blue-600));
         height: 2px;
         left: 0;
         position: absolute;
@@ -66,7 +66,7 @@
       }
 
       #knob {
-        background-color: var(--cr-slider-knob-color, --google-blue-600);
+        background-color: var(--cr-slider-knob-color, var(--google-blue-600));
         border: 0;
         border-radius: 50%;
         box-shadow: 0 1px 3px 0 rgba(0, 0, 0, 0.4);