Fix MWB shared style CSS variable

Bug: 1099917
Change-Id: I57bfd6b3cb8b5496663b5d5d9fcd75a9603d5e57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590627Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Roman Arora <romanarora@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836881}

ui/webui/resources/cr_elements/mwb_shared_vars.html

@@ -18,7 +18,7 @@
       html {
         --mwb-background-color: var(--google-grey-900);
         --mwb-icon-button-fill-color: var(--google-grey-refresh-300);
-        --mwb-icon-button-hover-background-color: rgba(255, 255, 255), 0.1);
+        --mwb-icon-button-hover-background-color: rgba(255, 255, 255, 0.1);
         --mwb-list-item-hover-background-color: rgb(55, 56, 58); /* #37383a */
         --mwb-list-item-selected-background-color: rgb(68, 69, 71); /* #444547 */
       }