WebUI Tab Strip: Make box-shadow outline on groups 100% opacity

As requested by UX.

Change-Id: I95847f267f74b199789f16ba1b5e2bc80e304747
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2076529
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745157}

chrome/browser/resources/tab_strip/tab_group.html

@@ -11,7 +11,7 @@
 
 #tabGroup {
   border-radius: 8px;
-  box-shadow: 0 0 0 1px rgba(var(--tabstrip-tab-group-color-rgb), .24);
+  box-shadow: 0 0 0 1px rgb(var(--tabstrip-tab-group-color-rgb));
   padding: var(--tabstrip-tab-spacing);
 }
 
@@ -22,7 +22,7 @@
 }
 
 #title {
-  background: rgba(var(--tabstrip-tab-group-color-rgb), 1);
+  background: rgb(var(--tabstrip-tab-group-color-rgb));
   border-radius: 4px;
   box-sizing: border-box;
   color: rgb(var(--tabstrip-tab-group-text-color-rgb));