WebUI Tab Strip: Make outlines on tab groups 2px

Change-Id: I50bb0446d8716a2f2f9372937f753571442fec49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398954Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805517}

chrome/browser/resources/tab_strip/tab_group.html

@@ -15,7 +15,7 @@
 
 #tabGroup {
   border-radius: 8px;
-  box-shadow: 0 0 0 1px rgb(var(--tabstrip-tab-group-color-rgb));
+  box-shadow: 0 0 0 2px rgb(var(--tabstrip-tab-group-color-rgb));
   padding: var(--tabstrip-tab-spacing);
 }