update_engine: Get OmahaRequestParams from SystemState

Can use Omaha request params from the singleton |SystemState| without
being passed as an argument to the constructor of |OmahaRequestAction|.

BUG=b:171829801
TEST=cros_workon_make --board reef --test update_engine

Change-Id: Iad0616db4e6e7aad2a2da3c5c0eb9cb9ea7d9329
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2543813
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/cros/omaha_request_action_unittest.cc b/cros/omaha_request_action_unittest.cc
index a3799b4..2e54a43 100644
--- a/cros/omaha_request_action_unittest.cc
+++ b/cros/omaha_request_action_unittest.cc
@@ -516,7 +516,7 @@
     fetcher->FailTransfer(tuc_params_.fail_http_response_code);
   }
   // This ensures the tests didn't forget to update |FakeSystemState| if they
-  // are not using the default request_params_.
+  // are not using the default |request_params_|.
   EXPECT_EQ(&request_params_, FakeSystemState::Get()->request_params());
 
   auto omaha_request_action =