[Fallback Icons] Refactor FallbackIconService to be a BrowserContext-level singleton

chrome/test/base/testing_profile.h

Index: chrome/test/base/testing_profile.h
diff --git a/chrome/test/base/testing_profile.h b/chrome/test/base/testing_profile.h
index 8db54eea1cb6ddf1d9fd6b1ddd14430a5ceee98e..b122728b7fe86407b51ecb81370800a0da6c56bf 100644
--- a/chrome/test/base/testing_profile.h
+++ b/chrome/test/base/testing_profile.h
@@ -164,6 +164,10 @@ class TestingProfile : public Profile {
 
   ~TestingProfile() override;
 
+  // Creates the fallbck icon service. Consequent calls would recreate the
+  // service.
+  void CreateFallbackIconService();
+
   // Creates the favicon service. Consequent calls would recreate the service.
   void CreateFaviconService();