Give gr-dropdown triggers a focus state

Restores the default focus outline for gr-dropdown triggers.

As a component of this change, the spacing in the main header has been
modified in order to make the focus rings appropriately fit the button
content.

Bug: Issue 6435
Change-Id: I38a4905c5bec2c3e90c2299ccdea1c6847d92e20

polygerrit-ui/app/elements/shared/gr-dropdown/gr-dropdown.html

@@ -50,7 +50,10 @@ limitations under the License.
         vertical-align: middle;
       }
       gr-button[link] {
-        padding: 1em 0;
+        padding: 0.5em;
+      }
+      gr-button[link]:focus {
+        outline: 5px auto -webkit-focus-ring-color;
       }
       ul {
         list-style: none;