Move UserCloudPolicyManagerTest to use mixin infrastructure

This moves UserCloudPolicyManagerTest to use mixin infrastructure to
set up test environment. This significantly reduces the inheritance
chain for these tests (to UserCloudPolicyManagerTest >
MixinBasedInProcessBrowserTest > InProcessBrowserTest), and makes the
test logic more trackable.

BUG=None

Change-Id: I60000a23c3b0d72cbe18fc101b48b96d09c3f603
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597174
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657490}

chrome/browser/chromeos/login/test/login_manager_mixin.cc

@@ -119,7 +119,8 @@ void LoginManagerMixin::CreatedBrowserMainParts(
 void LoginManagerMixin::SetUpOnMainThread() {
   test::UserSessionManagerTestApi session_manager_test_api(
       UserSessionManager::GetInstance());
-  session_manager_test_api.SetShouldLaunchBrowserInTests(false);
+  session_manager_test_api.SetShouldLaunchBrowserInTests(
+      should_launch_browser_);
   session_manager_test_api.SetShouldObtainTokenHandleInTests(false);
 }
 

chrome/browser/chromeos/login/test/login_manager_mixin.h

@@ -54,6 +54,11 @@ class LoginManagerMixin : public InProcessBrowserTestMixin {
   // Should be called before mixin SetUp() is called to take effect.
   void set_session_restore_enabled() { session_restore_enabled_ = true; }
 
+  // By default, LoginManagerMixin will set up user session manager not to
+  // launch browser as part of user session setup - use this to override that
+  // behavior.
+  void set_should_launch_browser(bool value) { should_launch_browser_ = value; }
+
   // Sets the list of default policy switches to be added to command line on the
   // login screen.
   void SetDefaultLoginSwitches(
@@ -96,6 +101,10 @@ class LoginManagerMixin : public InProcessBrowserTestMixin {
   bool session_restore_enabled_ = false;
   test::SessionFlagsManager session_flags_manager_;
 
+  // Whether the user session manager should skip browser launch steps for
+  // testing.
+  bool should_launch_browser_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(LoginManagerMixin);
 };