Fix ClearLogoOnSignOut flakiness

ClearLogoOnSignOut sometime fails due RunLoop exiting early.

The initial SetCachedLogo(nullptr) call from SignIn might not run.

Bug: 1003172
Change-Id: I46df0b4461f8e974160f22cbdf08ce1cba2280f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1799032Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Ilia Samsonov <isamsonov@google.com>
Cr-Commit-Position: refs/heads/master@{#696053}

components/search_provider_logos/logo_service_impl_unittest.cc

@@ -960,6 +960,8 @@ TEST_F(LogoServiceImplTest, DeleteExpiredCachedLogo) {
 TEST_F(LogoServiceImplTest, ClearLogoOnSignOut) {
   // Sign in and setup a logo response.
   signin_helper_.SignIn();
+  // |SetCachedLogo(nullptr)| task might not have run.
+  task_environment_.RunUntilIdle();
   Logo logo = GetSampleLogo(DoodleURL(), test_clock_.Now());
   SetServerResponse(ServerResponse(logo));