Ignore multiple calls to AndroidOverlay callbacks.

Occasionally, MojoAndroidOverlay will receive more than one failure
message from the server.  Ignore all but the first.

Bug: 861719
Change-Id: I38a98108bad29cacdce61863c0a8cd42eb8feb2e
Reviewed-on: https://chromium-review.googlesource.com/1161219Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580380}

media/base/android_overlay_config.h

@@ -65,8 +65,14 @@ struct MEDIA_EXPORT AndroidOverlayConfig {
   bool power_efficient = false;
 
   // Convenient helpers since the syntax is weird.
-  void is_ready(AndroidOverlay* overlay) { std::move(ready_cb).Run(overlay); }
-  void is_failed(AndroidOverlay* overlay) { std::move(failed_cb).Run(overlay); }
+  void is_ready(AndroidOverlay* overlay) {
+    if (ready_cb)
+      std::move(ready_cb).Run(overlay);
+  }
+  void is_failed(AndroidOverlay* overlay) {
+    if (failed_cb)
+      std::move(failed_cb).Run(overlay);
+  }
 
   ReadyCB ready_cb;
   FailedCB failed_cb;