Add null check in ArCoreJavaUtils::OnDrawingSurfaceDestroyed

`surface_destroyed_callback_` can potentially be null when
`ArCoreJavaUtils::OnDrawingSurfaceDestroyed` gets called.

Bug: 966984
Change-Id: I878ede8a419f95cd6bc083863f369f4054ce039b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648891
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Auto-Submit: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667360}

chrome/browser/android/vr/arcore_device/arcore_java_utils.cc

@@ -153,7 +153,9 @@ void ArCoreJavaUtils::OnDrawingSurfaceDestroyed(
     JNIEnv* env,
     const base::android::JavaParamRef<jobject>& obj) {
   DVLOG(1) << __func__ << ":::";
-  std::move(surface_destroyed_callback_).Run();
+  if (surface_destroyed_callback_) {
+    std::move(surface_destroyed_callback_).Run();
+  }
 }
 
 void ArCoreJavaUtils::OnRequestInstallArModuleResult(