Fix the flaky tests AppContextMenuTest.*

When the test exist, the data_decode process has't finished yet, so it
might cause fatal error:
data_decoder::ServiceProvider::Set() must be called before any
instances of DataDecoder can be used.

Call app_service_test().WaitForAppService() to add RunLoop to wait the
decode process finish to fix the flaky tests.

BUG=1115763

Change-Id: Ic03dd66eca615096b2e51b90b6d3a590fe7c5e8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2383593
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802991}

chrome/browser/ui/app_list/app_context_menu_unittest.cc

@@ -177,6 +177,9 @@ class AppContextMenuTest : public AppListTestBase,
   }
 
   void TearDown() override {
+    // Let any in-flight tasks finish, otherwise the test might flake
+    // (crbug.com/1115763).
+    app_service_test().WaitForAppService();
     menu_delegate_.reset();
     controller_.reset();
     menu_manager_.reset();