Close the shell when all activities are destroyed.

shell/android/main.cc

Index: shell/android/main.cc
diff --git a/shell/android/main.cc b/shell/android/main.cc
index f94068422071deb41207106c73a04cd306724d2a..eb31de581105afbfc50de179ce41af78a35524ed 100644
--- a/shell/android/main.cc
+++ b/shell/android/main.cc
@@ -341,6 +341,11 @@ static void BindShell(JNIEnv* env, jclass clazz, jint shell_handle) {
                                      mojo::MessagePipeHandle(shell_handle)))));
 }
 
+static void QuitShell(JNIEnv* env, jclass jcaller) {
+  g_internal_data.Get().shell_task_runner->PostTask(
+      FROM_HERE, base::MessageLoop::QuitWhenIdleClosure());
+}
+
 bool RegisterShellService(JNIEnv* env) {
   return RegisterNativesImpl(env);
 }