[Emoji] Update field trial test configuration

Enables it for Windows and ChromeOS

Bug: None
Change-Id: Id612d9893cc329dbd9d405c63261a049246e33d5
Reviewed-on: https://chromium-review.googlesource.com/1090408
Commit-Queue: Mathieu Perreault <mathp@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566692}

testing/variations/fieldtrial_testing_config.json

@@ -1441,7 +1441,9 @@
     "EnableEmojiContextMenu": [
         {
             "platforms": [
-                "mac"
+                "chromeos",
+                "mac",
+                "win"
             ],
             "experiments": [
                 {