WebUI: cr-slider, use em for label padding and border-radius

Bug: 980856
Change-Id: Iad9f20399ff575a22adeb91473784781a9e0a0a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776778Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Esmael Elmoslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691819}

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

@@ -144,13 +144,13 @@
 
       #label {
         background: var(--cr-slider-knob-color);
-        border-radius: 14px;
+        border-radius: .75em;
         bottom: 22px;
         color: white;  /* Same for dark and light mode. */
         font-size: 12px;
         line-height: 1.5em;
         opacity: 0;
-        padding: 0 8px;
+        padding: 0 .67em;
         position: absolute;
         transform: translateX(-50%);
         transition: opacity 80ms ease-in-out;