settings: Fix outline on "Add language" button after dialog closed

The outline was appearing even if the user used the mouse to click on
the button. Elsewhere in settings we only show outlines when the user
uses the keyboard. Update action-link CSS styles to check the global
focus-outline-visible style added by FocusOutlineManager.

I did this at the settings level, rather than for all action-links,
because I'm not sure that all consumers of action-link need or want
a dependency on FocusOutlineManager.

Bug: 995921
Test: no outline when button clicked with mouse, outline present when triggered with keyboard
Change-Id: Idb7d4e0f96bb70d1d24c63fbc760460198a08304
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762695Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689102}

chrome/browser/resources/settings/settings_shared_css.html

@@ -186,6 +186,13 @@
         min-height: inherit;
       }
 
+      /* Link buttons use FocusOutlineManager to only show outlines when focus
+       * was triggered by keyboard. */
+      :host-context(html:not(.focus-outline-visible))
+          .list-button[is='action-link'] {
+        outline: none;
+      }
+
       /* A row with two lines of text. Often the lower line will be .secondary.
        */
       .two-line {