Bug 1485809 - Fix race with compositor attachment r=geckoview-reviewers,droeh#geckoview-reviewers

We can receive GeckoSession.onCompositorDetached() before
GeckoSession.onCompositorReady() has run, so guard against this
by ensuring the compositor is attached in onCompositorReady().

Differential Revision: https://phabricator.services.mozilla.com/D16983

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

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

@@ -4438,6 +4438,10 @@ public class GeckoSession implements Parcelable {
             ThreadUtils.assertOnUiThread();
         }
 
+        if (!mAttachedCompositor) {
+            return;
+        }
+
         mCompositorReady = true;
 
         if (mController != null) {