Run async callbacks in uninstall-app browser_test

This fixes `unit_tests --enable-features=AppServiceAsh
--gtest_filter="AppListClientSearchResultsBrowserTest.UninstallSearchResult"`.
It fails before and passes after this commit.

Note that `unit_tests
--gtest_filter="AppListClientSearchResultsBrowserTest.UninstallSearchResult"`,
without AppServiceAsh enabled, passes both before and after this commit.

BUG=826982

Change-Id: Ib6d1b8f2bf8d4bc1a7c4558618f2c49587f11144
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1657072Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669071}

chrome/browser/ui/app_list/app_list_client_impl_browsertest.cc

@@ -302,6 +302,9 @@ IN_PROC_BROWSER_TEST_F(AppListClientSearchResultsBrowserTest,
   // Uninstall the extension.
   UninstallExtension(extension->id());
 
+  // Allow async callbacks to run.
+  base::RunLoop().RunUntilIdle();
+
   // We cannot find the extension any more.
   EXPECT_FALSE(search_controller->GetResultByTitleForTest(title));