[NTP] Fix unreadable text on MV notifications

When a custom background is set the theme is considered both "default"
and "dark" so the most visited notifications had their text-color set to
white. Mark the ".dark.default" rules to not apply to MD icons.

Bug: 854330
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I1aa73da1032addfe73db2f821bd9d945be27e5dd
Reviewed-on: https://chromium-review.googlesource.com/1115975Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570994}

chrome/browser/resources/local_ntp/local_ntp.css

@@ -599,11 +599,11 @@ html[dir=rtl] #mv-notice-x {
   padding: 0 16px;
 }
 
-.default-theme.dark #mv-msg {
+.default-theme.dark #mv-msg :not(.md-icons) {
   color: #fff;
 }
 
-.default-theme.dark #mv-notice-links span {
+.default-theme.dark #mv-notice-links :not(.md-icons) span {
   color: #fff;
 }