Use static_cast instead of reinterpret_cast for downcasting

static_cast is better suited for downcasting since it can check that the
types are related.

/chrome/browser/chromeos/login
This CL was uploaded by git cl split.

R=antrim@chromium.org

Change-Id: I295668a0311ddb164cd898569faf9041ca0e431b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435750
Auto-Submit: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811568}

chrome/browser/chromeos/login/users/avatar/user_image_manager_browsertest.cc

@@ -298,7 +298,7 @@ class UserImageManagerTestBase : public LoginManagerTest,
     const user_manager::User* user =
         user_manager::UserManager::Get()->GetActiveUser();
     ASSERT_TRUE(user);
-    UserImageManagerImpl* uim = reinterpret_cast<UserImageManagerImpl*>(
+    UserImageManagerImpl* uim = static_cast<UserImageManagerImpl*>(
         ChromeUserManager::Get()->GetUserImageManager(user->GetAccountId()));
     if (uim->job_.get()) {
       run_loop_.reset(new base::RunLoop);