[Tango->FCM] Enable FCMInvalidations featurein trial config for linux.

BUG=801985

Change-Id: I9fe5891fb8d30f869e2e53f6c915c89ee109d50a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1547848
Commit-Queue: Tatiana Gornak <melandory@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649195}

testing/variations/fieldtrial_testing_config.json

@@ -2021,7 +2021,9 @@
                 "windows",
                 "mac",
                 "chromeos",
-                "ios"
+                "ios",
+                "linux",
+                "android_webview"
             ],
             "experiments": [
                 {