Fix flake in some CloudPolicyTest tests after SimpleURLLoader conversion in r571184.

There was a race condition when the policy fetch request that's initiated in CloudPolicyTest::SetUpOnMainThread is cancelled becauase the test bodies start their own request (through RefreshPolicies). The first request seems to get the python test server in a bad state and it stops responding to requests afterwards.

The fix is to skip issuing the first request by temporarily unregistering CloudPolicyRefreshScheduler.

TBR=pmarko

Bug: 858883
Change-Id: I8c158ab3dffc2f38a7712a6806f9b8d28efbcfdb
Reviewed-on: https://chromium-review.googlesource.com/1119628
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571402}

chrome/browser/policy/cloud/cloud_policy_browsertest.cc

@@ -35,6 +35,7 @@
 #include "components/policy/core/browser/browser_policy_connector.h"
 #include "components/policy/core/common/cloud/cloud_policy_client.h"
 #include "components/policy/core/common/cloud/cloud_policy_constants.h"
+#include "components/policy/core/common/cloud/cloud_policy_refresh_scheduler.h"
 #include "components/policy/core/common/cloud/mock_cloud_policy_client.h"
 #include "components/policy/core/common/external_data_fetcher.h"
 #include "components/policy/core/common/policy_map.h"
@@ -235,6 +236,19 @@ class CloudPolicyTest : public InProcessBrowserTest,
 #endif  // defined(OS_CHROMEOS)
 
     ASSERT_TRUE(policy_manager->core()->client());
+
+    // The registration below will trigger a policy refresh (see
+    // CloudPolicyRefreshScheduler::OnRegistrationStateChanged). When the tests
+    // below call RefreshPolicies(), the first policy request will be cancelled
+    // (see CloudPolicyClient::FetchPolicy which will reset
+    // |policy_fetch_request_job_|). When the URLLoader implementation sees the
+    // SimpleURLLoader going away, it'll cancel it's request as well. This race
+    // sometimes causes errors in the Python policy server (|test_server_|).
+    // Work around this by removing the refresh scheduler as an observer
+    // temporarily.
+    policy_manager->core()->client()->RemoveObserver(
+        policy_manager->core()->refresh_scheduler());
+
     base::RunLoop run_loop;
     MockCloudPolicyClientObserver observer;
     EXPECT_CALL(observer, OnRegistrationStateChanged(_)).WillOnce(
@@ -260,6 +274,11 @@ class CloudPolicyTest : public InProcessBrowserTest,
     policy_manager->core()->client()->RemoveObserver(&observer);
     EXPECT_TRUE(policy_manager->core()->client()->is_registered());
 
+    // Readd the refresh scheduler as an observer now that the first policy
+    // fetch finished.
+    policy_manager->core()->client()->AddObserver(
+        policy_manager->core()->refresh_scheduler());
+
 #if defined(OS_CHROMEOS)
     // Get the path to the user policy key file.
     base::FilePath user_policy_key_dir;