ShellExtensionLoaderTest should use existing PrefService

ShellExtensionLoaderTest associates a new TestingPrefServiceSimple with
the BrowserContext, but instead should associate the existing
PrefService created by ExtensionsTest.

Creating its own TestingPrefServiceSimple, besides being unnecessary,
leads to invalid casts of its PrefRegistrySimple to a
PrefRegistrySyncable (breaking the Linux CFI bots).

TBR=rdevlin.cronin@chromium.org

Bug: 820647
Change-Id: I2ae62a396622d034c8c5b04bbe9dabb529b5bd25
Reviewed-on: https://chromium-review.googlesource.com/957871Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542302}

extensions/shell/browser/shell_extension_loader_unittest.cc

@@ -12,7 +12,6 @@
 #include "base/path_service.h"
 #include "components/crx_file/id_util.h"
 #include "components/keep_alive_registry/keep_alive_registry.h"
-#include "components/prefs/testing_pref_service.h"
 #include "components/user_prefs/user_prefs.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/test/test_utils.h"
@@ -95,7 +94,9 @@ class ShellExtensionLoaderTest : public ExtensionsTest {
 
     ExtensionsTest::SetUp();
     extensions_browser_client()->set_extension_system_factory(&factory_);
-    user_prefs::UserPrefs::Set(browser_context(), &testing_pref_service_);
+    // ExtensionsTest sets up the ExtensionPrefs, but we still need to attach
+    // the PrefService to the browser context.
+    user_prefs::UserPrefs::Set(browser_context(), pref_service());
     event_router_ = CreateAndUseTestEventRouter(browser_context());
   }
 
@@ -135,7 +136,6 @@ class ShellExtensionLoaderTest : public ExtensionsTest {
 
  private:
   MockExtensionSystemFactory<TestExtensionSystem> factory_;
-  TestingPrefServiceSimple testing_pref_service_;
 
   TestEventRouter* event_router_ = nullptr;  // Created in SetUp().