Bug 1506599 - Add @AnyThread annotation to GeckoWebExecutor r=agi

Differential Revision: https://phabricator.services.mozilla.com/D11652

--HG--
extra : moz-landing-system : lando

mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoWebExecutor.java

@@ -6,6 +6,7 @@
 
 package org.mozilla.geckoview;
 
+import android.support.annotation.AnyThread;
 import android.support.annotation.IntDef;
 import android.support.annotation.NonNull;
 
@@ -35,6 +36,7 @@ import org.mozilla.gecko.annotation.WrapForJNI;
  *     });
  * </pre>
  */
+@AnyThread
 public class GeckoWebExecutor {
     // We don't use this right now because we access GeckoThread directly, but
     // it's future-proofing for a world where we allow multiple GeckoRuntimes.