MD WebUI: Explicitly set text color in cr-action-menu

This fixes an issue where the menu would appear blank in MD Downloads
and Bookmarks.

BUG=708892
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2804623005
Cr-Commit-Position: refs/heads/master@{#462930}

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.html

@@ -9,7 +9,6 @@
         background-color: white;
         border: none;
         box-shadow: 0 2px 6px var(--paper-grey-500);
-        color: inherit;
         margin: 0;
         outline: none;
         padding: 8px 0;
@@ -23,7 +22,7 @@
         background: none;
         border: none;
         box-sizing: border-box;
-        color: inherit;
+        color: var(--paper-grey-900);
         font: inherit;
         min-height: 32px;
         padding: 0 24px;