[GN Build] Add a dependency on //third_party/mozilla to //ui/base on Mac.

BUG=none
TEST=none
R=avi@chromium.org

Review URL: https://codereview.chromium.org/603263003

Cr-Commit-Position: refs/heads/master@{#296889}

ui/base/BUILD.gn

@@ -528,8 +528,7 @@ component("base") {
 
   if (is_mac) {
     deps += [
-      #TODO(tfarina): port this third_party component to GN.
-      #"//third_party/mozilla",
+      "//third_party/mozilla",
     ]
 
     sources -= [