Fix vertical offset of menu links dropdown

In some browsers, and particularly Firefox, the menu links dropdown was
positioned lower than the menu item that opens it. When the mouse cursor
crosses this gap the dropdown would be hidden before the user could
click an option. Close the gap by repositioning the dropdown.

Bug: Issue 4812
Change-Id: I6640c6f8fb0e86456422e1509a38b10d2fb96e7d
diff --git a/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html b/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html
index 916726fd..3e30810 100644
--- a/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html
+++ b/polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html
@@ -64,7 +64,7 @@
         left: -.5em;
         padding: .5em 0;
         position: absolute;
-        top: 2.45em;
+        top: 2.4em;
         z-index: 1000;
       }
       .links li ul li a:link,