Add tests for Management API for supervised user extension install

Test that if a child account attempts to enable an extension then the
parent permission dialog opens.

This is a unit test because there are existing browser tests that
exercise the JS/C++ communication for the management API.

Bug: 1059999
Test: added to unit_tests

Change-Id: I7604d4805f23637a3eb862b1b3a5874f7fcbbb4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129032
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Dan S <danan@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756042}

extensions/browser/api/management/management_api.h

@@ -331,6 +331,14 @@ class ManagementAPI : public BrowserContextKeyedAPI,
     return supervised_user_service_delegate_.get();
   }
 
+  void set_delegate_for_test(std::unique_ptr<ManagementAPIDelegate> delegate) {
+    delegate_ = std::move(delegate);
+  }
+  void set_supervised_user_service_delegate_for_test(
+      std::unique_ptr<SupervisedUserServiceDelegate> delegate) {
+    supervised_user_service_delegate_ = std::move(delegate);
+  }
+
  private:
   friend class BrowserContextKeyedAPIFactory<ManagementAPI>;